Class 10 Chemistry Chapter 1 Periodic Table, Periodic Properties and Variations of Properties

Key Concept: Reactivity, Nature of Alkali Metals and Halogens

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion correctly states the trend in reactivity for alkali metals and halogens. Alkali metals become more reactive down the group due to decreasing ionisation energy ($IE$), which makes it easier for them to lose electrons. Halogens become less reactive down the group because their electron affinity ($EA$) decreases, making it harder for them to gain electrons. The reason correctly explains the assertion by linking the reactivity trends to ionisation energy and electron affinity, both of which are key factors influencing the chemical behavior of these elements.

Key Concept: Comparison of Alkali Metals and Halogens

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion is true because alkali metals have only one valence electron, which they can easily lose due to low effective nuclear charge, resulting in low ionization energy. Halogens, on the other hand, have seven valence electrons and high electronegativity, making it difficult to remove an electron, thus requiring higher ionization energy. The reason correctly explains why alkali metals have lower ionization energy compared to halogens.

Key Concept: Comparison of Alkali Metals and Halogens

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Alkali metals are indeed good conductors of electricity because they have one valence electron that can move freely, facilitating electrical conduction. This matches the assertion (A). The reason (R) correctly explains why alkali metals are good conductors: their single valence electron is loosely bound, allowing it to be delocalized in a metallic lattice, which supports the conduction of electricity. Hence, both assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Mass number, neutron/proton ratio

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Sodium has an atomic number ($Z$) of 11 (odd). According to the given syllabus pattern for odd-$Z$ elements, the mass number ($A$) should be $2Z + 1 = 2(11) + 1 = 23$. This matches the assertion. Additionally, the reason correctly explains that this adjustment maintains stability by keeping the $n/p$ ratio around 1. Both are true and logically connected.

Key Concept: Mass Number and Atomic Number Relationship

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
- **Step 1:** Sodium has an atomic number ($Z$) of 11, which is odd. According to the syllabus, for elements with odd atomic numbers, the mass number ($A$) is typically given by $A = 2Z + 1$.
- **Step 2:** Substituting $Z = 11$ into the formula, we get:
$A = 2 \times 11 + 1 = 23$
- **Step 3:** The assertion states that the mass number of Sodium is 23, which matches our calculation.
- **Step 4:** The reason explains the general rule for elements with odd atomic numbers, which directly supports the assertion in this case.

Key Concept: Mass Number

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Sodium ($Na$) has an atomic number of 11, which means it has 11 protons. The mass number of sodium is given as 23. Using the formula for mass number: $Mass\ number\ (A) = No.\ of\ protons\ (p) + No.\ of\ neutrons\ (n)$. Here, $A = 23$, so $No.\ of\ neutrons = A - p = 23 - 11 = 12$. Therefore, the Assertion is true because the mass number is indeed 23. The Reason is also true because the mass number is equal to the sum of protons and neutrons (11 protons + 12 neutrons = 23). Moreover, the Reason correctly explains the Assertion since the definition of mass number is the sum of protons and neutrons.

Key Concept: relation between atomic number and mass number of light elements, stable n/p ratio

d) Assertion is false, but Reason is true.
[Solution Description]
Step 1: Understand the Assertion.
The assertion states that for lighter elements with even atomic number ($Z$), the mass number ($A$) is always exactly twice $Z$.
Step 2: Examine the syllabus example.
According to the syllabus, most lighter elements with even $Z$ (e.g., $He$, $RG$) follow $A = 2Z$, but there are exceptions like °Be and AG Ar.
Step 3: Analyze the Reason.
The reason claims that stability arises from an n/p ratio around 1 for lighter elements with even $Z$. This aligns with the syllabus note on stability conditions.
Step 4: Verify the Assertion against the syllabus.
Since the Assertion says "always" but exceptions exist (e.g., °Be, AG Ar), it is not entirely true.
Step 5: Assess correctness of Reason.
The Reason correctly identifies the stability condition (n/p $\approx$ 1) for these elements, as mentioned in the syllabus.
Conclusion: The Assertion is false because it ignores exceptions, but the Reason is true based on the syllabus.

Key Concept: Relation between Atomic Number and Mass Number

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that for $^{23}Na$, the mass number ($A = 23$) is twice its atomic number ($Z = 11$) plus one, i.e., $A = 2(11) + 1 = 23$. This aligns correctly with the reason, which explains the general trend for lighter elements with odd atomic numbers. Therefore, both the assertion and reason are true, and the reason accurately explains the assertion.

Key Concept: Atomic number and mass number relation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The atomic number (Z) of Helium is 2, which means it has 2 protons. According to the given syllabus, for light elements with an even atomic number, the mass number (A) is approximately twice the atomic number, i.e., A = 2Z. Helium's mass number is indeed approximately 4, which matches the assertion. The reason correctly explains why the mass number is twice the atomic number for elements like Helium. Hence, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Electron Affinity Trends, Group and Periodic Variation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Electron affinity is the energy released when an electron is added to a neutral gaseous atom. Across a period, electron affinity increases due to decreasing atomic size and increasing nuclear charge. However, down a group, atomic size increases more than nuclear charge, leading to a decrease in electron affinity.
In Group 17, fluorine has a smaller atomic size than chlorine, causing stronger inter-electronic repulsions in its compact valence shell. This makes it harder for fluorine to accept an additional electron compared to chlorine, which has a larger atomic size and less inter-electronic repulsion. Thus, fluorine has a lower electron affinity than chlorine (-349 kJ/mol for chlorine vs. -328 kJ/mol for fluorine).
Hence, Assertion (A) is true as fluorine indeed has lower electron affinity than chlorine, and Reason (R) correctly explains this phenomenon by pointing out the effect of atomic size and inter-electronic repulsion.

Key Concept: Electron Affinity Trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that fluorine has a lower electron affinity than chlorine, which is true based on the given syllabus. This happens because fluorine's small size causes strong inter-electronic repulsions in its compact 2p subshell, making it less effective at attracting an additional electron compared to chlorine's larger 3p subshell. The reason correctly explains this phenomenon by linking fluorine's smaller size to higher repulsion and reduced electron attraction. Therefore, both assertion and reason are correct, and the reason explains the assertion perfectly.

Key Concept: Variation in Electron Affinity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that halogens have the highest electron affinity in their respective periods, which is true because they require only one more electron to achieve a stable noble gas configuration. The reason explains this by stating that halogens have a strong attraction for an additional electron due to their high nuclear charge and small atomic size. This is also true and correctly explains why halogens have the highest electron affinity.

Key Concept: Trends in ionisation energy, Exceptions

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion (A): The first ionisation energy of Nitrogen $1402\ \text{kJ mol}^{-1}$
is indeed higher than that of Oxygen $1314\ \text{kJ mol}^{-1}$. This is because Nitrogen has a half-filled p-orbital configuration $1s^2\,2s^2\,2p^3$, which provides extra stability. Removing an electron from this stable configuration requires more energy.
Reason (R): Half-filled and fully filled orbitals are more stable due to symmetry and exchange energy. This stability results in higher ionisation energy for elements with such configurations.
Since the assertion is true and the reason correctly explains why nitrogen's ionisation energy is higher than oxygen’s, both statements are true, and the reason is the correct explanation for the assertion.

Key Concept: Trends in Ionisation Energy

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion (A) states that the ionisation energy of nitrogen N is higher than that of oxygen O. This is true because nitrogen has a half-filled 2p subshell $2p^3$, which gives it extra stability, making it harder to remove an electron. Oxygen, on the other hand, has an electron configuration of $2p^4$, so removing one electron leads to a more stable half-filled configuration, requiring less energy. Thus, the ionisation energy of nitrogen is indeed higher.
Reason (R) states that nitrogen has a stable half-filled electronic configuration $2p^3$), while oxygen does not. This is also true as nitrogen's half-filled configuration provides extra stability due to symmetry and exchange energy. Oxygen does not have a half-filled configuration in its ground state $2p^4$ but achieves it after losing one electron.
The reason correctly explains why the assertion is true because the stability of nitrogen's half-filled configuration directly contributes to its higher ionisation energy compared to oxygen.

Key Concept: Trends in Ionisation Energy

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In the 3rd period, sodium (Na) has an ionisation energy of $496 \ kJ \ mol^{-1}$ while magnesium (Mg) has $737 \ kJ \ mol^{-1}$.
Step 1: Compare atomic sizes: Sodium has larger atomic size than magnesium because as we move left to right across a period, the atomic size decreases due to increased nuclear charge pulling electrons closer.
Step 2: Nuclear charge comparison: Magnesium has higher nuclear charge ($12$) than sodium ($11$), so its outermost electron is held more tightly.
Thus, assertion (A) is true as Na’s I.E. is indeed lower, and reason (R) correctly explains why—due to Na’s larger atomic size and lesser nuclear charge compared to Mg.

Key Concept: Gradation in Physical Properties, Melting/Boiling Points Trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that iodine has a higher melting point than fluorine due to stronger van der Waals forces caused by its larger atomic size. This is true because, for non-metals like halogens (fluorine, chlorine, bromine, iodine), the melting and boiling points increase down the group. The reason provided is also correct: as we move down the group, the atomic size increases, leading to stronger van der Waals forces (intermolecular forces), which require more energy to overcome, resulting in higher melting and boiling points. Therefore, the reason correctly explains the assertion.

Key Concept: Gradation in Physical Properties

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, for non-metals, the melting and boiling points increase as we move down the group. Iodine is below bromine in Group 17 (halogens), so its melting point should be higher than that of bromine. The assertion states this correctly, and the reason provides the correct explanation based on the periodic trend given in the syllabus.

Key Concept: Gradation in Physical Properties

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, the melting and boiling points of metals decrease on going down the group. This is because as we move down the group, the atomic size increases, reducing the strength of metallic bonds due to larger interatomic distances. Hence, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Ionization Potential, Electronegativity, Atomic Size

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
First, let's analyze the Assertion:
- Across a period, ionization potential increases from left to right.
- Phosphorus ($P$) is to the right of Magnesium ($Mg$) in period 3.
- Therefore, $P$ indeed has a higher ionization potential than $Mg$.
So, the Assertion is true.
Now, let's analyze the Reason:
- Atomic size decreases across a period due to increased nuclear pull.
- Since $P$ is to the right of $Mg$, its atomic size is smaller.
- Hence, the Reason is also true.
Does the Reason correctly explain the Assertion?
- Yes, because the smaller atomic size of $P$ means electrons are held more tightly by the nucleus, leading to higher ionization potential.
- Thus, the Reason directly explains the Assertion.

Key Concept: Ionization Potential and Atomic Size

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Across a period, ionization potential increases from left to right due to increasing nuclear charge and decreasing atomic size. Sodium ($Na$) is located to the left of magnesium ($Mg$) in the third period, so $Na$ has a larger atomic size and lower effective nuclear charge than $Mg$. This makes it easier to remove an electron from $Na$ compared to $Mg$, hence its ionization potential is lower. The reason correctly explains the assertion as the larger atomic size directly reduces the nuclear pull on the valence electron, lowering the ionization potential.

Key Concept: Atomic Size and Electronegativity

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Assertion: Atomic size decreases from left to right across a period because the effective nuclear charge increases, pulling electrons closer to the nucleus. This is a correct trend as per the syllabus.
Reason: Electronegativity increases across a period due to stronger nuclear attraction for electrons, which is also true. However, this does not directly explain why atomic size decreases. The correct explanation for atomic size decreasing is the increasing nuclear charge, not electronegativity.
Therefore, both assertion and reason are true, but the reason does not correctly explain the assertion.

Key Concept: Chemical Reactivity Trends, Reactivity of Non-metals

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion states that fluorine is more reactive than chlorine due to its higher tendency to accept electrons. This is true because reactivity in non-metals depends on their ability to gain electrons. Fluorine, being at the top of Group 17, has a smaller atomic radius and higher electronegativity, making it easier for it to attract and gain electrons.
The reason states that the atomic radius decreases as we move down Group 17, which is incorrect. Actually, the atomic radius increases as we move down the group due to the addition of new electron shells. However, despite the larger atomic radius, the effective nuclear charge decreases, reducing the ability to gain electrons. The correct explanation for fluorine's higher reactivity is its smaller atomic size and higher electronegativity, not the decrease in atomic radius down the group.
Therefore, the Assertion is true, but the Reason is false.

Key Concept: Chemical Reactivity Trends in Periods

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that sodium Na is more reactive than magnesium Mg in the third period. This is true because sodium is a group 1 element and has a single valence electron, which it can lose more easily compared to magnesium, a group 2 element with two valence electrons.
The reason states that sodium can lose electrons more easily due to its lower ionization energy. This is also correct because group 1 elements have lower ionization energies than group 2 elements in the same period. Moreover, the lower ionization energy directly explains why sodium is more reactive than magnesium, as reactivity in metals depends on their tendency to lose electrons.
Thus, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Chemical Reactivity Trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that sodium is more reactive than magnesium, which is true because sodium belongs to group 1 and magnesium belongs to group 2. As per the syllabus, group 1 elements can lose electrons more easily compared to group 2 elements, making them more reactive. Therefore, the reason correctly explains why sodium is more reactive than magnesium.

Key Concept: Non-metallic character, Atomic size and Nuclear charge

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that non-metallic character decreases down Group 17 (Fluorine to Iodine). This is correct because as we move down the group, the atomic size increases due to the addition of new electron shells. Though the nuclear charge also increases, the effect of increasing atomic size dominates, leading to weaker nuclear pull on valence electrons and reduced tendency to gain electrons (non-metallic nature decreases).
The reason correctly explains that atomic size increases down the group, weakening the nuclear pull on valence electrons and decreasing the tendency to gain electrons. Therefore, the reason is the correct explanation for the assertion.

Key Concept: Trends in Non-metallic Character

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that the non-metallic character increases across a period from left to right. This is correct because as we move from left to right in a period, the number of valence electrons increases, making it easier for atoms to gain electrons to complete their octet.
The Reason explains that across a period, the atomic size decreases while the nuclear charge increases. This is also correct because a smaller atomic size results in stronger nuclear pull on the valence electrons, increasing the tendency to gain electrons. Moreover, an increased nuclear charge further enhances this tendency.
The Reason correctly explains why the Assertion is true, as both involve the factors (atomic size and nuclear charge) that contribute to the increase in non-metallic character across a period.

Key Concept: Trends in Non-Metallic Character

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion states that fluorine has a higher non-metallic character than chlorine. This is true because as per the syllabus, non-metallic character decreases down the group due to increasing atomic size and decreasing nuclear pull effect. Reason states that the tendency to gain electrons decreases down the group for non-metals, which is also true and correctly explains why fluorine is more non-metallic than chlorine as it lies above chlorine in the same group.

Key Concept: Ionization Energy, Atomic Size

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that sulfur has higher ionization energy than phosphorus, which is correct because ionization energy generally increases across a period due to increasing nuclear charge and decreasing atomic size. The reason correctly explains this trend by stating sulfur has a smaller atomic size than phosphorus because of increased effective nuclear charge. Thus, both are true and the reason is the correct explanation.

Key Concept: Electronegativity Trend Across a Period

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion (A) is true: Electronegativity indeed increases from left to right across a period due to increasing effective nuclear charge and decreasing atomic radius. Reason (R) is also true: The increase in nuclear charge combined with reduced atomic size enhances the nucleus's pull on bonding electrons. Additionally, Reason (R) correctly explains Assertion (A), as these factors directly lead to higher electronegativity values.

Key Concept: Electronegativity trend across a period

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, electronegativity increases across a period (left to right). This is because as we move left to right, the effective nuclear charge experienced by valence electrons increases due to increased protons in the nucleus, while the atomic size decreases due to the same reason. Both these factors lead to stronger attraction of the shared electron pair towards the nucleus, increasing electronegativity. Thus, both Assertion and Reason are true, and Reason correctly explains Assertion.

Key Concept: Atomic Size Trends, Isoelectronic Ions

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Both $Na^+$ and $F^-$ have 10 electrons, so they are isoelectronic. In isoelectronic species, the size depends on the nuclear charge: higher nuclear charge means smaller size. $Na^+$ has 11 protons while $F^-$ has only 9, so the effective nuclear charge is higher in $Na^+$. This stronger pull from the nucleus makes $Na^+$ smaller than $F^-$. Therefore, the Assertion is true, and the Reason correctly explains why $Na^+$ is smaller than $F^-$.

Key Concept: Atomic Size Trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, inert gas atoms like Neon (Ne) have a bigger size compared to other elements in the same period because their outermost shell is complete. This leads to maximum electronic repulsions, increasing the atomic radius. Fluorine (F), being a halogen, has a smaller size due to higher effective nuclear charge pulling electrons closer. Therefore, the Assertion (A) is correct as Ne's size is indeed larger than F's. The Reason (R) accurately explains why inert gases have larger atomic sizes compared to other elements in the same period.

Key Concept: Trends in Atomic Size

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the given syllabus, a cation is always smaller than its parent atom because the loss of electron(s) results in protons outnumbering electrons. This increases the effective nuclear charge, pulling the remaining electrons closer to the nucleus. Therefore, both Assertion and Reason are true, and Reason correctly explains Assertion.

Key Concept: Electronegativity, Ionisation Energy

d) Assertion is false, but Reason is true.
[Solution Description]
Assertion (A): Fluorine has higher electronegativity than chlorine because, across the period, electronegativity increases from left to right due to increasing nuclear charge and decreasing atomic size. However, the assertion mentions "across the period," but fluorine and chlorine are in the same group (Group 17), not period. Hence, Assertion (A) is misleading as it incorrectly refers to the trend across the period for comparison between group elements.
Reason (R): This statement correctly states that ionisation energy decreases down a group due to increased atomic size, reducing the effective nuclear attraction on the outermost electron. Thus, Reason (R) is true.
Therefore, Assertion (A) is false as it misrepresents the trend, while Reason (R) is true independently.

Key Concept: Atomic Size and Metallic Character

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the atomic size of sodium (Na) is larger than that of magnesium (Mg), which is correct. Across a period from left to right, the atomic size decreases due to an increase in effective nuclear charge, pulling electrons closer to the nucleus. Sodium (Group 1) has a larger atomic size than magnesium (Group 2).
The reason states that sodium has fewer protons (11) compared to magnesium (12), resulting in weaker effective nuclear charge and larger atomic size. This is accurate and correctly explains why the atomic size of Na is larger than Mg.
Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Periodic Trends - Electronegativity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that electronegativity increases across a period from left to right. This is true because as we move across a period, the effective nuclear charge increases due to more protons being added to the nucleus while electrons are added to the same shell. The reason given is that the nuclear charge increases across a period, leading to stronger attraction for electrons. This is also true and correctly explains why electronegativity increases across a period. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Shells and Valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
- Fluorine (F) has 7 valence electrons (electronic configuration: $2,8,7$) and requires 1 more electron to complete its octet. Hence, its valency is 1 (acceptor).
- Oxygen (O) has 6 valence electrons (electronic configuration: $2,6$) and requires 2 more electrons to complete its octet. Hence, its valency is 2 (acceptor).
- The Reason correctly explains why elements have different valencies based on their position in the periodic table and their valence electron count.
- Both Assertion and Reason are true, and the Reason logically justifies the Assertion using the octet rule principle from the syllabus.

Key Concept: Shells and Valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, as we move down a given group, the number of electrons in the outermost shell (valence electrons) remains the same. Since valency depends on the number of valence electrons, it also remains constant within a group. Thus, both Assertion (A) and Reason (R) are true, and Reason correctly explains Assertion.

Key Concept: Shells and Valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, moving down a group adds one shell per period. For example, fluorine (F) is in the second period with 2 shells, chlorine (Cl) is in the third period with 3 shells, bromine (Br) in the fourth period has 4 shells, and so on. This shows that the number of shells increases sequentially down a group and matches the period number. Therefore, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Valency, Variation in Valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to **Note 1** in the syllabus, if the number of electrons present in the outermost shell are 6, then their valency is $8 - 6 = 2$. This is because the atom needs 2 more electrons to complete its octet and achieve stability. Therefore, Assertion (A) is correct.
Reason (R) states that the number of electrons needed to complete the octet determines the valency, which is also correct as per the syllabus definition of valency. Moreover, the reason correctly explains why the valency is 2 for 6 valence electrons because the atom requires 2 more electrons to complete the octet.
Thus, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Valency and Group Trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Nitrogen is in group 15, so it has 5 valence electrons. According to the syllabus, the valency for elements with 5, 6, or 7 valence electrons is calculated as $8 - \text{number of valence electrons}$. Therefore, nitrogen's valency is $8 - 5 = 3$. This means the Assertion (A) is true. The Reason (R) correctly explains why nitrogen's valency is 3 as per the given formula, so both are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Valency in Groups

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The valency of an element is determined by the number of electrons in its outermost shell (valence electrons). For Group 1 elements, there is 1 valence electron, so their valency is 1 (donating 1 electron). For Group 17 elements, there are 7 valence electrons, so their valency is $8 - 7 = 1$ (accepting 1 electron to complete the octet). Thus, both Assertion and Reason are true, and the Reason correctly explains why both groups have a valency of 1.

Key Concept: Periods, Number of Shells

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, the modern periodic table has seven periods, and elements in the same period share the same number of electron shells. For instance, all second-period elements (like lithium, beryllium, etc.) have two electron shells, and all third-period elements (like sodium, magnesium, etc.) have three electron shells. This confirms that Assertion (A) is true.
The Reason (R) states that the period number equals the number of electron shells, which is also a correct statement from the syllabus. For example, sodium (period 3) has three shells, and calcium (period 4) has four shells. Moreover, since the period number directly determines the number of shells, Reason (R) correctly explains Assertion (A).

Key Concept: Relationship Between Period Number and Shells

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion: The electronic configuration of Chlorine is $2, 8, 7$, which indicates electrons are distributed in three shells (K, L, M). Therefore, Chlorine belongs to the third period as per the syllabus.
Reason: According to the syllabus, the period number corresponds to the number of shells. Hence, the reason correctly explains why the assertion is true.

Key Concept: Number of Shells

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
- The assertion states that sodium is in the third period due to having three electron shells, which is correct as its electronic configuration is $2, 8, 1$ with three shells.
- The reason correctly explains the assertion by stating the general rule that the number of shells determines the period of an element, which applies to all elements including sodium. Therefore, both are true and the reason properly justifies the assertion.

Key Concept: Cause of Periodicity, Electronic Configuration

d) Assertion is false, but Reason is true.
[Solution Description]
Let's break down the Assertion and Reason:
- **Assertion (A):** It states that the similarity in chemical properties among elements of the same group is due to identical atomic masses. However, this is incorrect. Chemical properties are determined by the electronic configuration, particularly the number of valence electrons, not atomic masses. For instance, halogens (Group 17) have varying atomic masses but share similar chemical properties due to having seven valence electrons.
- **Reason (R):** This correctly states that elements in the same group exhibit similar chemical properties because they have the same number of electrons in their outermost shells. This is consistent with the syllabus, which emphasizes the recurrence of similar electronic configurations as the cause of periodicity.
Since the Assertion is false and the Reason is true, the correct option is the one where Assertion is false but Reason is true.

Key Concept: Cause of Periodicity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that elements in the same group have similar chemical properties, which is true because chemical properties depend on the number of valence electrons. The reason explains that this similarity is due to the recurrence of similar electronic configurations in the outermost shells, which directly determines the chemical behavior of elements. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Cause of Periodicity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, periodicity occurs due to the repetition of similar electronic configurations in the outermost shell across periods and groups. This leads to similar chemical properties among elements in the same group because they share the same number of valence electrons. Therefore, both Assertion (A) and Reason (R) are true, and Reason correctly explains the Assertion.

Key Concept: Periodicity, Shells and Valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the boiling points of halogens increase as we move down the group due to an increase in the number of electron shells. This is true because halogens are diatomic molecules and their boiling points depend on van der Waals forces, which strengthen with increasing molecular size (more electron shells).
The reason given is that an increase in the number of electron shells leads to stronger van der Waals forces. This is also correct because more electron shells mean larger atomic/molecular size and greater polarizability, enhancing van der Waals interactions.
Moreover, the reason correctly explains the assertion since the variation in boiling points among halogens is indeed due to changes in van der Waals forces caused by differences in the number of electron shells.

Key Concept: Periodicity and Electronic Configuration

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion is true because elements in the same group share similar chemical properties due to having the same number of valence electrons, which determine their reactivity and bonding behavior. The Reason is also true as it correctly explains the cause of periodicity—similar electronic configurations (same number of valence electrons). Since the Reason directly explains the Assertion, the correct option is both are true with the Reason being the right explanation.

Key Concept: Periodicity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description] The assertion states that elements in the same group show similar chemical properties, which is true as per the syllabus. The reason given is that elements in the same group have the same number of valence electrons, which is also true. Moreover, according to the syllabus, having the same number of valence electrons directly causes the similarity in chemical properties. Therefore, the reason correctly explains the assertion.

Key Concept: Periodicity, Group and Period Determination, Representative Elements

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
First, let's analyze the Assertion (A): Copper (Cu) has an atomic number of 29, and its electronic configuration is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^{10}$. Its outermost shell is the 4th shell, which contains only 1 electron ($4s^1$). However, according to the modern periodic table classification, Group 11 elements have 1 or 2 electrons in their outermost shell. Therefore, the assertion that Copper (Cu) is placed in Group 11 due to having one valence electron is correct.
Now, the Reason (R): The position of an element in the modern periodic table is indeed determined by its electronic configuration. The period is determined by the number of shells, and the group is determined by the number of valence electrons. This reason is true and correctly explains why Copper (Cu) is placed in Group 11.

Key Concept: Periodic Table - Groups and Periods

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that sodium is placed in the third period of the modern periodic table. This is true because sodium's atomic number is 11, and its electronic configuration is $2, 8, 1$, meaning it has three electron shells (K, L, M). According to the syllabus, the number of shells determines the period of an element, so sodium belongs to the third period.
The Reason states that the number of shells in a sodium atom is three, which determines its period. This is also true and correctly explains why sodium is placed in the third period. Thus, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Groups and Periods

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
- According to the syllabus, the period of an element is determined by the number of electron shells in its atom.
- Sodium has the electronic configuration $2, 8, 1$, indicating it has three electron shells ($n = 1, 2, 3$). Thus, it belongs to the third period.
- The Reason correctly explains why sodium is placed in the third period, as the number of shells directly determines the period number.

Key Concept: Noble Gases properties, Reactivity comparison with other groups

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that noble gases do not form compounds under normal conditions because of their complete octet configuration. This is correct as noble gases have a stable electron configuration ($ns^2 np^6$), which makes them highly unreactive. The reason compares noble gases with alkali metals and halogens, stating that they have the highest ionization energy in their respective periods. While it's true that noble gases have high ionization energies, this alone does not explain their chemical inertness directly. The primary reason for their inertness is their stable octet configuration, not just high ionization energy. Therefore, both statements are true, but the reason does not correctly explain the assertion.

Key Concept: Chemical Inertness of Noble Gases

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that noble gases are inert and do not react easily with other elements under normal conditions. This is true because noble gases like Helium, Neon, Argon, etc., have fully filled valence shells (complete octet except Helium, which has a duplet). The reason given is that noble gases have a complete octet in their outermost shell, which makes them stable. This is also true. Moreover, the reason correctly explains why noble gases are inert—having a complete octet leads to high stability and low reactivity. Thus, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Properties of Noble Gases

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Noble gases like Helium (He), Neon (Ne), etc., are part of Group 18 and have a completely filled valence shell (octet or duplet for Helium). This stable electronic configuration makes them highly unreactive or inert as they do not gain, lose, or share electrons easily. The assertion correctly states that noble gases are chemically inert. The reason accurately explains this by pointing out the complete octet configuration. Thus, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Electronegativity, Reactivity Trend in Halogens

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Fluorine has the highest electronegativity among halogens, which allows it to attract electrons most strongly, making it the best oxidizing agent (Assertion is true). The reactivity of halogens decreases from fluorine to iodine because their ability to gain electrons (electronegativity) reduces down the group as atomic size increases, shielding the nucleus' pull on valence electrons (Reason is true and correctly explains Assertion.

Key Concept: Halogens Reactivity Trend

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Fluorine is indeed more reactive than chlorine, as reactivity decreases down the group for halogens. This is because fluorine has a higher electronegativity than chlorine, which means it attracts electrons more strongly and thus reacts more readily to complete its octet. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Halogens: Group 17 - Non-metals, Reactive, Form Salts

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that halogens are highly reactive non-metals, which is true as they readily react with metals and non-metals to form compounds like salts. The Reason explains that halogens have seven valence electrons and tend to gain one electron to complete their octet, which is also true. The high reactivity of halogens is directly due to their tendency to gain an electron to achieve a stable octet configuration, making the Reason the correct explanation of the Assertion.

Key Concept: Atomic size, Chemical reactivity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the chemical reactivity of alkali metals increases down the group, which is true because as we move down Group 1, the atomic size increases. This results in the outermost electron being farther from the nucleus, decreasing the effective nuclear attraction. Consequently, ionization energy decreases, making it easier for the metal to lose its valence electron and react. Therefore, the reason correctly explains why the reactivity increases down the group.

Key Concept: Reactivity Trend in Alkali Metals

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that potassium reacts more vigorously with water than sodium due to its lower ionization energy. This is true because reactivity of alkali metals increases down the group as they lose their valence electron more easily. The reason correctly explains this trend by stating that ionization energy decreases down the group, leading to increased reactivity. Thus, both assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Reactivity Trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The reactivity of alkali metals increases down the group due to decreasing ionization energy. Sodium ($Na$) is below lithium ($Li$) in Group 1, so it has a larger atomic size and lower ionization energy. This means sodium loses its valence electron more easily than lithium, leading to more vigorous reactions. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Transition elements, Noble gases

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion is true as transition elements indeed exhibit variable oxidation states due to their incomplete d-orbitals, allowing them to lose different numbers of electrons. The reason is also true since noble gases have a stable electronic configuration with completely filled outermost orbitals, making them chemically inert. However, the reason does not explain why transition elements exhibit variable oxidation states; it only explains the inertness of noble gases. Thus, both statements are true, but the reason does not correctly explain the assertion.

Key Concept: Alkali Metals and Alkalis

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Group 1 elements (alkali metals) form strong alkalis with water, which is true as they react vigorously to produce hydroxides like $NaOH$ or $KOH$, which are strong bases. The reason explains that this is due to the release of hydroxide ions ($OH^-$) during the reaction, making the solution strongly alkaline. This correctly explains the assertion because the high reactivity of alkali metals with water leads to the formation of strong bases, confirming the release of hydroxide ions as the cause of alkalinity.

Key Concept: Special Groups

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that alkali metals form strong alkalis with water, which is correct because alkali metals (Group 1) react vigorously with water to form hydroxides (strong bases). The reason given is also true as alkali metals have a single electron in their outermost shell, which they lose easily during reactions. Additionally, this property (easily losing the outer electron) explains why they form strong bases with water, making the reason a correct explanation for the assertion.

Key Concept: Electronic Configuration, Zero Valency

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Assertion (A) states that noble gases like Xenon (Xe) can form compounds under certain conditions despite having zero valency. This is true because noble gases, such as Xenon, can form compounds with highly electronegative elements (e.g., Fluorine or Oxygen) under extreme conditions due to the availability of d-orbitals for hybridization, though their natural state is inert. Reason (R) states that the outermost shell of noble gases is completely filled, making them chemically inert. This is also true as the complete octet (or duplet for Helium) results in zero valency and inertness under normal conditions. However, Reason (R) does not explain why noble gases like Xenon can sometimes form compounds; it only explains their general inertness. Thus, both statements are true, but Reason (R) is not the correct explanation of Assertion (A).

Key Concept: Noble Gases & Zero Valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that noble gases are chemically inert and do not form compounds under normal conditions. This is true because noble gases like Helium (He), Neon (Ne), etc., have complete valence electron configurations, which makes them stable. The reason given is that noble gases have completely filled valence shells, which prevents them from gaining, losing, or sharing electrons under normal conditions. This is also true and correctly explains the assertion. Thus, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Noble Gases - Zero Valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that noble gases have zero valency, which is true because noble gases do not donate, accept or share electrons due to their stable electronic configuration. The reason given is that their outermost electron shell is completely filled, which is also true and directly explains why they have zero valency. Therefore, both assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Valency patterns, Group trends

c) Assertion is true, but Reason is false.
[Solution Description]
The assertion states that the valency of Group IVA elements is 4 due to having four valence electrons, which is correct according to the syllabus. However, the reason claims that these elements can either lose all four valence electrons or gain four more to complete their octet, which is incorrect. In reality, Group IVA elements typically share electrons (covalent bonding) rather than losing or gaining them entirely, as losing four electrons would require excessive energy and gaining four is highly improbable due to electron-electron repulsion. Thus, while the assertion is true, the reason is false.

Key Concept: Valency in Groups and Periods

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Group 1 and Group 17 elements have a valency of 1 due to their need to gain or lose one electron for stability. This is correct: Group 1 elements (e.g., Na, K) have 1 valence electron and lose it to form $M^+$ ions, while Group 17 elements (e.g., F, Cl) have 7 valence electrons and gain 1 electron to form $X^-$ ions, achieving an octet. The reason explains the mechanism behind this observation: Group 1 elements donate electrons (electropositive), whereas Group 17 elements accept electrons (electronegative). Hence, the reason correctly explains the assertion.

Key Concept: Group 17 Valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, Group 17 elements (halogens) have 7 valence electrons, and their valency is determined by the formula $8 - \text{number of valence electrons}$. Here, chlorine has 7 valence electrons, so its valency is $8 - 7 = 1$. The Assertion states that chlorine has a valency of 1, which is true based on the given calculation in the Reason. The Reason also correctly explains why chlorine has a valency of 1. Therefore, both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Group number and valency, Valence electrons, Halogens

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion states that all elements in Group 17 have a valency of 1 because they can gain one electron to complete their octet. This is correct as per Note 1 in the syllabus which states that elements with 7 valence electrons have a valency of $8 - 7 = 1$.
Reason claims that the valency is determined by the number of electrons needed to achieve a stable electronic configuration. This is also correct, as valency depends on the number of electrons required to fill the outermost shell (octet rule), whether by gaining, losing, or sharing.
Furthermore, Reason correctly explains why Group 17 elements have a valency of 1, since they require 1 more electron to complete their octet.

Key Concept: Relation between group number and valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Group 14 elements have a valency of 4 because they possess four valence electrons. This is correct as per the syllabus, which mentions that if the number of valence electrons is 1, 2, 3, or 4, the valency equals that number. Group 14 elements indeed have four valence electrons, so their valency is 4.
The reason given is that valency is determined by the number of electrons in the outermost shell, and Group 14 elements have four valence electrons. This is also true and directly explains why the assertion is correct. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Relation between group number and valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that elements in the same group have the same valency, which is true because valency is determined by the number of valence electrons. The reason explains that valency depends on the number of valence electrons, and this is also correct since elements in the same group have the same number of valence electrons. Therefore, the reason correctly explains the assertion.

Key Concept: Periodic Table, Valency

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Assertion Statement Analysis:
- The assertion claims that Group 14 elements have a valency of 4 due to their ability to lose or gain 4 electrons for stability.
- According to the syllabus, Group 14 elements indeed have a valency of 4. However, the reasoning provided is incomplete. While they *can* gain 4 electrons (resulting in a valency of 4 as per $8 - 4 = 4$), they can also *lose* these 4 electrons directly (valency = 4). The assertion oversimplifies by implying both mechanisms are equally dominant without context, but the core claim about valency is correct.
Reason Statement Analysis:
- The reason states that Group 14 elements have 4 valence electrons and calculates valency based on electron gain ($8 - 4 = 4$).
- This is partially correct per syllabus notes, but it ignores the alternative mechanism of losing electrons (direct valency = 4). Thus, the reason is factually true but not exhaustive.
Conclusion:
Both Assertion and Reason are true, but the Reason only explains one aspect (electron gain) and not the complete picture (electron loss). Hence, it is not the correct explanation of the Assertion.

Key Concept: Valency Trends in Periodic Table

c) Assertion is true, but Reason is false.
[Solution Description]
Assertion (A) is true because as we move left to right across a period, the valency increases from 1 to 4 (Group 1 to Group 14) and then decreases from 3 to 1 (Group 15 to Group 17). Reason (R) is false because the number of valence electrons does not decrease after Group 14; it continues to increase up to Group 18 (noble gases). The valency decreases beyond Group 14 because the atoms tend to gain electrons to complete their octet (Groups 15-17 have 5-7 valence electrons respectively, so they accept 3-1 electrons, making their valency $8 - n$ where $n$ is the number of valence electrons).

Key Concept: Valency Determination

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, when an element has 5 electrons in its outermost shell, its valency is calculated as $8 - 5 = 3$. Hence, Assertion (A) is true. Additionally, the Reason (R) correctly explains the method to determine valency for such elements (groups 15-17), so it is also true and explains Assertion (A).

Key Concept: Periodic Properties, Shells and Valency

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that atomic size decreases across a period due to increasing effective nuclear charge. This is correct because, as we move left to right in a period, the number of protons in the nucleus increases while electrons are added to the same shell. The increased positive charge attracts the outermost electrons more strongly, pulling them closer and reducing the atomic radius. The reason states that the increase in effective nuclear charge reduces the atomic radius by pulling electrons closer to the nucleus, which correctly explains the assertion. Hence, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Periodicity Explanation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that elements in the same group show similar chemical properties, which is true as per the syllabus. The reason given is also correct because elements in the same group indeed have the same number of valence electrons (electrons in the outermost shell), which determines their chemical behavior. Therefore, the reason correctly explains the assertion.

Key Concept: Periodic Properties

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that elements in the same group show similar chemical properties, which is true as per the syllabus. The reason given is that they have the same number of electrons in their outermost shell, which is also true and directly explains why the assertion is correct. Similar outermost electron configuration leads to similar chemical behavior, as chemical properties depend on the valence electrons.

Key Concept: Isotopes, Noble Gases

d) Assertion is false, but Reason is true.
[Solution Description]
The Assertion states that isotopes of an element occupy different positions in the periodic table, which is false because isotopes of an element are placed at the same position due to having the same atomic number. The Reason correctly explains that isotopes have the same number of protons (same atomic number) but different numbers of neutrons. Since the Assertion is false while the Reason is true, the correct option is (d).

Key Concept: Isotopes and Periodic Table

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that isotopes occupy the same position in the Periodic Table, which is true because isotopes are variants of the same element and thus share the same atomic number, placing them in the same position. The reason given is that isotopes have the same atomic number but different mass numbers, which is also correct and directly explains why they occupy the same position in the Periodic Table. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Position of isotopes in periodic table

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that isotopes of an element are placed in the same position in the periodic table, which is true because the periodic table arranges elements based on their atomic number. The reason given is that isotopes have the same atomic number but different mass numbers, which is also true and correctly explains why isotopes occupy the same position in the periodic table.

Key Concept: Atomic Number, Mass Number, Stability

d) Assertion is false, but Reason is true.
[Solution Description]
The assertion states that $^{14}N$ has a mass number equal to twice its atomic number plus one ($A = 2Z + 1$), which would imply $A = 15$ since $Z = 7$. However, the mass number of $^{14}N$ is 14, not 15. Thus, the assertion is false. The reason correctly identifies the general trend for elements with odd atomic numbers but also notes exceptions like $^{14}N$, making it true.

Key Concept: Atomic Number and Mass Number Relation

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
- Sodium ($Na$) has an atomic number ($Z$) of 11, meaning it has 11 protons.
- According to the syllabus, for light elements with odd atomic numbers, the mass number ($A$) is typically given by $A = 2Z + 1$. Plugging in $Z = 11$, we get $A = 2(11) + 1 = 23$.
- The mass number is the sum of protons and neutrons: $A = p + n \Rightarrow 23 = 11 + n \Rightarrow n = 12$.
- Thus, both the Assertion and Reason are true, and the Reason correctly explains why the mass number of $^{23}Na$ is 23.

Key Concept: Atomic Number and Mass Number

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the atomic number of an element is equal to the number of protons in its nucleus, which is true based on the syllabus definition of atomic number (Z). The reason states that the atomic number uniquely identifies an element because no two elements have the same number of protons, which is also true according to the syllabus. Moreover, the reason correctly explains the assertion since the uniqueness of the atomic number arises from the fact that it counts the protons in the nucleus, making it a distinctive property for each element.

Key Concept: Mendeleev’s Periodic Law, Modern Periodic Law

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Mendeleev’s periodic table failed to place isotopes correctly because it was based on atomic mass. This is true since isotopes of an element have identical atomic numbers but different masses, causing inconsistency in Mendeleev’s classification. The reason correctly explains this limitation by highlighting that isotopes defy the very basis (atomic mass) of Mendeleev’s law. Thus, both the assertion and reason are true, and the reason accurately explains the assertion.

Key Concept: Basis of Classification in Periodic Tables

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the Modern Periodic Table uses atomic number for classification, while Mendeleev's table used atomic mass. This is correct as per the syllabus. The reason explains why atomic number is more suitable: it determines electronic configuration and thus chemical properties, unlike atomic mass, which can have anomalies (e.g., Argon and Potassium). Therefore, both the assertion and reason are true, and the reason correctly justifies the assertion.

Key Concept: Basis of Classification

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Mendeleev’s periodic table was based on atomic mass, which is true as per the syllabus. The reason states that the modern periodic table is based on atomic number, which also correctly addresses the limitation of Mendeleev’s classification by using atomic number. Hence, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Electron affinity, Electronegativity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion (A) states that fluorine has a lower electron affinity than chlorine despite having higher electronegativity. This is true because although fluorine is more electronegative, its small atomic size causes significant inter-electronic repulsion, which reduces the net attraction for an extra electron compared to chlorine. Thus, fluorine's electron affinity is less than chlorine's.
Reason (R) explains that the small atomic size of fluorine leads to strong inter-electronic repulsions, reducing the effective attraction for an additional electron. This is also true and directly explains why fluorine has lower electron affinity despite higher electronegativity.
Therefore, both Assertion and Reason are true, and Reason correctly explains the Assertion.

Key Concept: Electron Affinity and Electronegativity Trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that fluorine has a lower electron affinity than chlorine, which is true. According to the syllabus, fluorine has lower electron affinity due to its small size causing strong inter-electronic repulsions in the compact 2p subshell, making it harder for an additional electron to be accommodated compared to chlorine, where the incoming electron can be added more easily to the larger 3p subshell with less repulsion. The reason correctly explains why fluorine has lower electron affinity than chlorine by pointing out its small size and increased repulsion effects.

Key Concept: Electron Affinity Trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, fluorine has a lower electron affinity than chlorine because of its small atomic size leading to higher inter-electronic repulsions when an extra electron is added. This makes it harder for fluorine to accept an electron compared to chlorine. Therefore, both the Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Ionisation energy trends across period and down group

d) Assertion is false, but Reason is true.
[Solution Description]
Assertion: The given statement compares the ionisation energies of nitrogen ($1314\ kJ\ mol^{-1}$) and oxygen ($2080\ kJ\ mol^{-1}$). From the data, we observe that the assertion is false because the ionisation energy of nitrogen is indeed less than that of oxygen. However, this contradicts the general trend where ionisation energy increases across a period (left to right).
Reason: The reason states that nitrogen has a half-filled p-orbital configuration providing extra stability. This is true as nitrogen's electronic configuration is $1s^2 2s^2 2p^3$ with half-filled p-orbitals. Half-filled orbitals are more stable due to symmetry and exchange energy.
Analysis: Although the reason is correct about nitrogen’s stability, it does not explain why oxygen has higher ionisation energy than nitrogen. The actual explanation for oxygen’s higher ionisation energy is increased effective nuclear charge and decreased atomic size compared to nitrogen, making it harder to remove an electron from oxygen.
Conclusion: The assertion is false, but the reason is true.

Key Concept: Trends in Ionisation Energy

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the ionisation energy of nitrogen N is higher than that of oxygen O in the 2nd period. This is true because nitrogen has a half-filled 2p subshell $1s^2\,2s^2\,2p^3$, which gives it additional stability due to symmetrical electron distribution and exchange energy. Oxygen, on the other hand, has the configuration $1s^2\,2s^2\,2p^4$, where pairing of electrons in one orbital leads to increased electron-electron repulsion, making it easier to remove an electron.
The reason correctly explains why nitrogen has higher ionisation energy: its half-filled $2p$ subshell results in greater stability and resistance to electron removal. Therefore, both the assertion and the reason are true, and the reason is the correct explanation for the assertion.

Key Concept: Ionisation Energy Trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the ionisation energy of Mg is higher than Na, which is correct as per the given data (Mg: 737 kJ mol$^{-1}$, Na: 496 kJ mol$^{-1}$). The reason explains the general trend across a period, where increasing nuclear charge reduces atomic size and increases ionisation energy. Since the reason correctly explains why Mg has higher ionisation energy than Na, both assertion and reason are true, and the reason is the correct explanation.

Key Concept: Non-metallic character, Trends in periodic table

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that non-metallic character increases across a period due to decreasing atomic size and increasing nuclear charge. This is correct because as we move from left to right in a period, the atomic size decreases (due to increased effective nuclear charge pulling electrons closer), and the nuclear charge increases. Both factors enhance the ability of an atom to gain electrons, thereby increasing its non-metallic character.
The Reason explains that a smaller atomic size leads to a stronger nuclear pull on valence electrons, facilitating electron gain. This is also correct and directly explains why the Assertion holds true. Thus, both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Non-metallic character trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the non-metallic character increases across a period from left to right, which is correct according to the syllabus. This is due to increasing nuclear pull and decreasing atomic size, making it easier for elements to gain electrons. The reason provided correctly explains this trend by mentioning the decrease in atomic size and increase in nuclear charge, both of which contribute to higher non-metallic character. Therefore, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Trend in Non-metallic Character Across Period and Down Group

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that non-metallic character increases across a period, which is correct because as we move from left to right in a period, the effective nuclear charge increases while the atomic size decreases. This results in a stronger pull on the valence electrons, making it easier for the element to gain electrons, thus increasing its non-metallic character. The reason correctly explains this by mentioning the decrease in atomic size and increase in nuclear charge, which directly contribute to the observed trend.

Key Concept: Metallic character trends: across period & down group

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that lithium (Li) has a lower metallic character than sodium (Na). This is true because as we move down Group 1 (alkali metals), the metallic character increases due to greater atomic size and weaker nuclear attraction on valence electrons. Sodium (Na) is below lithium (Li) in the group, confirming this trend. The reason explains that down a group, the tendency to lose electrons increases due to increasing atomic size and shielding effect, which is accurate and correctly justifies the assertion. Hence, both assertion and reason are true, and the reason explains the assertion correctly.

Key Concept: Metallic Character Trend

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Moving across a period from left to right, the atomic number increases, which means the nuclear charge (protons in the nucleus) increases. The increased nuclear charge pulls electrons closer to the nucleus, reducing the atomic size. As a result, it becomes harder for the atom to lose its outermost electrons, decreasing its metallic character. The reason correctly explains why the assertion is true since both are directly linked by the effect of increasing nuclear pull on electron loss tendency.

Key Concept: Metallic Character Trend

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Across a period, the nuclear charge increases due to an increase in protons, which pulls electrons closer to the nucleus, reducing atomic size. As a result, elements find it harder to lose electrons, decreasing their metallic character. Hence, the assertion is true. The reason correctly explains the assertion because increased nuclear pull directly affects atomic size and electron loss tendency.

Key Concept: Atomic radius trends, Isoelectronic ions

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
To analyze this assertion-reason pair:
- Both $Na^+$ and $F^-$ have 10 electrons (isoelectronic with neon).
- Nuclear charge of $Na^+$ = +11 (protons), while $F^-$ = +9.
- Higher nuclear charge in $Na^+$ pulls electrons closer, reducing its size compared to $F^-$.
- Thus, Assertion (A) is true.
- Reason (R) correctly explains the observation because increased nuclear charge indeed leads to stronger attraction and smaller size.

Key Concept: Atomic Size Trends

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the atomic radius of Sodium (Na) is larger than that of Magnesium (Mg). According to the syllabus, atomic size decreases across a period due to increased nuclear charge pulling electrons closer to the nucleus. Sodium (Group 1) has a larger atomic radius (186 pm) compared to Magnesium (Group 2, 160 pm), as given in the syllabus.
The reason correctly explains the trend: as we move from left to right in a period, the increasing nuclear charge reduces atomic size. Thus, both Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: Atomic Size Trend

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the atomic size of sodium (Na) is greater than that of chlorine (Cl), which is correct as per the trend across the third period: $\text{Na} (186 \text{pm}) > \text{Mg} (160 \text{pm}) > \ldots > \text{Cl} (99 \text{pm})$. The reason explains that this decrease in size across a period is due to increasing nuclear charge, which is also correct and directly explains the assertion. Thus, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Periodicity, Metallic Character, Non-metallic Character

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that metallic character increases down the group and decreases across a period. This is true because as we move down a group, the atomic size increases due to the addition of new shells, which reduces the attraction between the nucleus and the outermost electrons, making it easier to lose electrons (increasing metallic character). Across a period, the atomic size decreases while the effective nuclear charge increases, making it harder to lose electrons (decreasing metallic character). The reason correctly explains the assertion by mentioning the variation in effective nuclear charge, which directly influences the metallic character.
Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Periodic Trends in Atomic Size

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion (A) states that atomic size decreases across a period, which is correct because as we move left to right in a period, the number of protons (atomic number) increases, increasing the effective nuclear charge. This stronger attraction pulls the electrons closer to the nucleus, reducing atomic size. Reason (R) correctly explains Assertion (A) by stating that increased effective nuclear charge is responsible for this trend.

Key Concept: Periodicity in Properties

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that elements in the same group have similar chemical properties, which is true as per the syllabus. The reason given is that elements in the same group have the same number of electrons in their outermost shell, which is also correct. Furthermore, the syllabus explicitly mentions that the recurrence of similar electronic configuration (same number of valence electrons) causes periodicity and leads to similar chemical properties within a group. Therefore, the reason correctly explains the assertion.

Key Concept: Electronegativity, Periodicity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Electronegativity is the tendency of an atom to attract shared electrons in a chemical bond. It increases across a period due to increasing effective nuclear charge and decreases down a group due to increasing atomic size. Fluorine (F) is in the same group as chlorine (Cl) but is placed above it in the periodic table. Fluorine has a smaller atomic radius compared to chlorine, which means the outermost electrons are closer to the nucleus and experience a stronger pull from the nuclear charge. Moreover, fluorine's effective nuclear charge is higher because there are fewer electron shells shielding the nuclear charge compared to chlorine. Both these factors contribute to fluorine having a higher electronegativity than chlorine. Thus, the Assertion is true, and the Reason correctly explains why fluorine is more electronegative than chlorine.

Key Concept: Atomic Size and Periodicity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
**Assertion Explanation**:
Atomic size decreases across a period because the effective nuclear charge (positive charge experienced by valence electrons) increases. This happens as protons are added to the nucleus without adding new electron shells, pulling electrons closer.
**Reason Explanation**:
The reason correctly states that the nuclear charge increases across a period due to more protons. The shielding effect (repulsion between inner-shell electrons and valence electrons) remains nearly the same because no additional electron shells are added.
**Conclusion**:
Both Assertion and Reason are true, and the Reason accurately explains why the atomic radius decreases across a period.

Key Concept: Metallic Character

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Metallic character refers to the tendency of an element to lose electrons. Down a group, the atomic size increases due to the addition of new electron shells, which results in weaker attraction between the nucleus and the valence electrons. This makes it easier for the atom to lose electrons, hence increasing its metallic character. Therefore, both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A).

Key Concept: Electronic Configuration, Periodicity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
In d-block elements, as we move across a period, the number of protons increases, increasing the nuclear charge. However, the d-electrons being added to the penultimate shell do not shield the outer electrons effectively due to their diffuse nature. This results in a higher effective nuclear charge acting on the valence electrons, leading to a decrease in atomic size across the period. Hence, both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains (A).

Key Concept: Electronic Configuration and Block Classification

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion states that Iron (Fe) is a d-block element because its outermost electrons occupy the d-orbital. This is true as d-block elements include those where the last electron enters the d-orbital. The Reason provides the electronic configuration of Fe as $[\text{Ar}] 3d^6 4s^2$, which shows that the outermost electrons indeed occupy the 3d orbital. Thus, both Assertion and Reason are true, and the Reason correctly explains the Assertion.

Key Concept: s-block elements

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that sodium Na is an s-block element because its outermost electron enters the s-orbital. This is correct because sodium has an atomic number of 11 with an electronic configuration of 2, 8, 1, where the last electron occupies the 3s orbital. The reason explains that all Group 1 and Group 2 elements are s-block elements since their outermost electrons enter the s-orbital, which is also correct. Furthermore, the reason correctly justifies why sodium is classified as an s-block element.

Key Concept: Electronic configuration of transition elements, f-block elements

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Cerium (Ce) has atomic number 58 and belongs to the lanthanide series (f-block). Its electronic configuration is $[Xe] 4f^1 5d^1 6s^2$. This indicates that one electron enters the 4f orbital and another enters the 5d orbital before completing the 6s orbital. This behavior occurs because for f-block elements, the $(n-2)f$ orbitals are filled before the $(n-1)d$ orbitals due to their lower energy levels compared to $(n-1)d$ orbitals. However, this filling order can sometimes lead to exceptions where electrons occupy higher-energy orbitals first, as seen in Ce's case. Therefore, the Assertion correctly describes Ce's configuration, and the Reason accurately explains why f-block elements fill electrons in $(n-2)f$ before $(n-1)d$, but does not fully account for the exception observed in Ce's configuration.

Key Concept: s-Block Elements

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Group 1 elements form stronger alkalis with water than Group 2 elements, which is true because alkali metals (Group 1) react vigorously with water to form strong bases like NaOH, while alkaline earth metals (Group 2) form weaker bases like Mg(OH)$_2$. The reason given is that Group 1 elements have one valence electron which is more easily lost than the two valence electrons of Group 2 elements. This is also true and correctly explains why Group 1 elements are more reactive and form stronger alkalis—their single valence electron is easier to lose, leading to more vigorous reactions.

Key Concept: Electronic Configuration and Block Classification

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that sodium (Na) is an s-block element due to its outermost electronic configuration being $3s^1$, which is correct as s-block elements have their outermost electrons in the s-orbital ($ns^{1-2}$). The reason explains that elements with the outermost electron in the s-orbital are classified as s-block elements, which is a true statement and directly supports the assertion. Therefore, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Groups, Periods, and Metallic Character

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that the metallic character decreases across a period due to decreasing tendency to lose electrons. This is correct because metals are characterized by their ability to lose electrons easily.
The Reason explains that the effective nuclear charge increases across a period due to more protons and constant shielding, making electron loss difficult. This correctly explains why the metallic character decreases.
Hence, both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Key Concept: Groups and Periods

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion is true because sodium ($Na$) indeed has an electronic configuration of $2, 8, 1$, meaning it has one electron in its outermost shell, placing it in Group 1. The reason is also true because all elements in Group 1 share the common property of having one electron in their outermost shell, making them alkali metals. Furthermore, the reason correctly explains why sodium belongs to Group 1. Therefore, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Groups and Periods

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to the syllabus, Group 1 elements are alkali metals and have one valence electron. Sodium's electronic configuration is 2, 8, 1, which shows it has one valence electron. Hence, it belongs to Group 1. The Reason correctly explains why Sodium is in Group 1.

Key Concept: Atomic number, Mass number, Relation between A and Z for light elements

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that for oxygen (Z = 8), the mass number is 16 (twice the atomic number). This is correct because $A = 2Z = 16$.
The reason generalizes this rule for all "light elements" with even atomic numbers, stating that $A \approx 2Z$. This is consistent with the syllabus, which specifies exceptions like Be (Z = 4, A = 9) and Ar (Z = 18, A = 40). However, for most light elements with even Z, the relation holds true.
Since the reason correctly explains why oxygen follows the $A \approx 2Z$ rule, both statements are true, and the reason is the correct explanation of the assertion.

Key Concept: Atomic Number and Electronic Configuration

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the element with atomic number 17 has an electronic configuration of $2, 8, 7$ and belongs to Group 17 (VIIA) of the periodic table. According to the syllabus, this is correct because the atomic number determines the electronic configuration, which in turn determines the group and period of the element.
The reason states that the group number of an element is determined by the number of electrons in its outermost shell. This is also correct as per the syllabus, where it is mentioned that valency (and hence group number) depends on the number of electrons in the outermost shell.
Furthermore, the reason correctly explains the assertion because the electronic configuration ($2, 8, 7$) shows 7 electrons in the outermost shell, which places it in Group 17 (VIIA). Thus, both are true, and the reason is the correct explanation for the assertion.

Key Concept: Atomic Number and Mass Number

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Assertion (A): True. The mass number ($A$) is defined as the sum of protons ($Z$) and neutrons ($n$). This is a fundamental concept from the syllabus: $A = Z + n$.
Reason (R): True. The atomic number ($Z$) determines the electronic configuration, which in turn determines the position of the element in the periodic table. However, this does not explain why the mass number is the sum of protons and neutrons. The reason is true but unrelated to the assertion.
Therefore, both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

Key Concept: Modern Periodic Table, Group and Period Classification

a)
[Solution Description]
The electronic configuration of calcium (Ca) with atomic number 20 is $2,8,8,2$. This means calcium has 2 valence electrons. According to the modern periodic table classification, elements are placed in groups based on the number of valence electrons. Since calcium has 2 valence electrons, it is correctly placed in Group 2. Therefore, both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation for Assertion (A).

Key Concept: Periodicity

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Assertion (A) states that atomic size decreases across a period, which is true because as we move from left to right in a period, the number of protons increases, increasing the nuclear charge. This stronger positive charge attracts the electrons more strongly, reducing the atomic radius. Reason (R) correctly explains this by stating that the nuclear charge increases, pulling electrons closer, which directly accounts for the decrease in atomic size. Hence, both Assertion and Reason are true, and Reason is the correct explanation of Assertion.

Key Concept: Groups in Modern Periodic Table

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that sodium is placed in Group 1 of the modern periodic table, which is correct as per the syllabus. The reason given is that sodium has one electron in its outermost shell, which is also correct. According to the syllabus, elements are grouped based on the number of valence electrons in their outermost shell. Since sodium's electronic configuration is $2,8,1$, it has one valence electron, justifying its placement in Group 1. Therefore, the reason correctly explains the assertion.

Key Concept: Anomalous pairs, position of isotopes, hydrogen

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the atomic size of neon is greater than fluorine in the same period. According to the syllabus, inert gases like neon indeed have larger atomic sizes due to their complete outer shells and maximum electron-electron repulsion. The reason correctly explains why neon has a larger atomic size than fluorine: it arises from the repulsion between electrons in filled orbitals, which expands the electron cloud. Thus, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: General Example

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Hydrogen indeed has one valence electron, which justifies its placement in Group 1. However, hydrogen also exhibits properties similar to halogens, making its classification anomalous. The assertion is true, but the reason alone does not fully explain its anomalous nature.

Key Concept: Mendeleev's periodic table, prediction of new elements

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that Mendeleev’s periodic table predicted new elements like gallium and germanium correctly because it was based on atomic masses. This is true as Mendeleev left gaps for undiscovered elements and accurately predicted their properties. However, the reason claims that periodicity is inherently linked to atomic number, which is also true but contradicts the assertion since Mendeleev’s table was based on atomic masses, not atomic numbers. Therefore, the reason does not explain the assertion.

Key Concept: Prediction of New Elements

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Mendeleev arranged elements in order of increasing atomic mass but left gaps where he believed new elements would be discovered. These gaps were later filled by elements such as gallium (eka-aluminium) and germanium (eka-silicon). The Reason correctly explains the Assertion because Mendeleev used the periodic trends to predict the properties of these elements, which matched closely with their actual discovery.

Key Concept: Merits of Mendeleev's Periodic Table

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Mendeleev left gaps in his periodic table for undiscovered elements, which is true as he anticipated new elements would be discovered to fit those gaps. The reason given is that the properties of these undiscovered elements could be predicted based on trends, which is also true and directly explains why Mendeleev left the gaps—to accommodate future discoveries with predictable properties. Hence, both the assertion and reason are correct, and the reason correctly explains the assertion.

Key Concept: Basis of classification: Atomic mass, Trends for light elements

d) Assertion is false, but Reason is true.
[Solution Description]
Let us analyze both the assertion and reason step-by-step.
1. **Assertion**: The statement mentions that the mass numbers of $^9Be$ ($A=9$, $Z=4$) and $^{36}Ar$ ($A=36$, $Z=18$) do not follow the general trend $A = 2Z$. For $^9Be$, $2Z = 8 \neq A = 9$, and for $^{36}Ar$, $2Z = 36 = A = 36$. Here, only $^9Be$ deviates while $^{36}Ar$ follows the trend. Thus, the assertion is **partially correct but misleading** as it incorrectly includes $^{36}Ar$.
2. **Reason**: The reason states that stability requirements for light elements cause deviations from general mass-number trends. This is true. Elements with even $Z$ usually have $A = 2Z$, but exceptions like $^9Be$ occur due to stability constraints related to neutron-proton ratios ($n/p$), which must be around 1 for light nuclei. Hence, the reason is **correct**.
Since Assertion is partially incorrect (as $^{36}Ar$ does NOT deviate) while Reason is completely true, the correct option is:

Key Concept: Atomic Mass Classification

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Assertion (A): Mendeleev arranged elements based on atomic mass, and he noticed that elements with similar properties fell under the same group due to periodic repetition in properties. This is true as per the syllabus.
Reason (R): The periodic law stated by Mendeleev explicitly connects atomic mass and properties, making this statement true. However, the Reason does not directly explain why Mendeleev chose atomic mass for classification; it only restates the observation from which the periodic law was derived.
Thus, both statements are true, but Reason does not correctly explain Assertion.

Key Concept: Atomic Mass and Classification

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Mendeleev's periodic table was based on atomic masses, which is true as he arranged elements in order of increasing atomic mass. The reason states that the properties of elements are periodic functions of their atomic masses, which is also true and matches Mendeleev's periodic law. Furthermore, the reason correctly explains why Mendeleev chose atomic mass as the basis for classification.

Key Concept: Mendeleev’s Periodic Table, Limitations of Mendeleev’s Periodic Table

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Mendeleev's periodic table could not assign a fixed position to hydrogen. This is true because hydrogen has unique properties: like alkali metals, it can lose an electron to form H$^+$, and like halogens, it can gain an electron to form H$^-$. Due to this dual nature, hydrogen did not fit neatly into any single group in Mendeleev's classification system.
The reason states that hydrogen exhibits properties similar to both alkali metals and halogens, which is also true. However, this ambiguity in properties is precisely why Mendeleev's table couldn't fix hydrogen's position. Hence, the reason correctly explains the assertion.

Key Concept: Mendeleev’s Periodic Table

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Mendeleev's periodic table could not explain the positions of isotopes, which is true as per the syllabus. The reason given is that isotopes have the same atomic number but different atomic masses, and Mendeleev’s classification was based on atomic mass, which is also true. Since the inability to place isotopes correctly in the table is directly due to Mendeleev's reliance on atomic mass, the reason correctly explains the assertion.

Key Concept: Defects of Mendeleev’s Periodic Table

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Mendeleev’s periodic table failed to position isotopes correctly, which is true as isotopes share the same atomic number (same element) but differ in atomic mass. The reason explains why this happened—Mendeleev arranged elements based on atomic mass rather than atomic number, leading to confusion in placing isotopes. Both statements are correct, and the reason provides a valid explanation for the assertion.

Key Concept: Dobereiner's Triads, Limitations of early models

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Dobereiner's triad classification had limited applicability because elemental properties do not consistently follow atomic mass trends. This is true since triads could only be formed for a few groups (e.g., Li, Na, K; Ca, Sr, Ba), and most elements did not fit this pattern. The reason provides the specific limitation: the atomic mass condition (middle element's mass ≈ average of the other two) was not satisfied universally. Thus, both statements are true, and the reason correctly explains the assertion.

Key Concept: Limitations of early models

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
Dobereiner's Triads grouped elements with similar chemical properties into sets of three, where the atomic mass of the middle element was roughly the average of the other two. This worked for some triads like Lithium (Li), Sodium (Na), Potassium (K). However, it failed for many other elements because the relationship was not universally applicable due to inconsistent atomic mass differences. Thus, Assertion (A) is true. The Reason (R) correctly explains why Dobereiner's Triads failed—the atomic mass differences were not uniform across all proposed triads, which led to exceptions and incompleteness in classification. Therefore, both Assertion and Reason are true, and Reason explains the Assertion correctly.

Key Concept: Limitations of early models

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that Mendeleev's periodic table could not explain the position of isotopes, which is true because his table was based on atomic mass, and isotopes of an element have the same atomic number but different atomic masses. The reason given is also correct as isotopes indeed have the same atomic number but different atomic masses, and this directly explains why Mendeleev's model failed to account for them. Thus, both the assertion and reason are true, and the reason correctly explains the assertion.

Key Concept: Dobereiner's Triads, Newlands' Law of Octaves

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Dobereiner's triads state that the atomic mass of the middle element is approximately the average of the other two elements in the triad. The atomic masses are: Li = 7, Na = 23, K = 39. The average of Li and K is $(7 + 39)/2 = 23$, which matches Na's atomic mass. The assertion is true. Newlands' Law of Octaves states that every eighth element has similar properties to the first, but it was only applicable up to calcium (Ca), unlike Dobereiner's triads, which were not limited by atomic mass. The reason correctly highlights a limitation of Newlands' law but does not explain why Dobereiner's assertion is valid. Thus, both are true, but the reason does not explain the assertion.

Key Concept: Dobereiner's Triads

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that in Dobereiner’s triad of Li, Na, and K, the atomic mass of Na is approximately the average of the atomic masses of Li and K.
Given:
- Atomic mass of Li = 7
- Atomic mass of Na = 23
- Atomic mass of K = 39
The average of Li and K is $\frac{7 + 39}{2} = 23$, which matches the atomic mass of Na. Thus, the Assertion is true.
The Reason states that the properties of the middle element in a triad are intermediate between those of the other two elements. While this is generally correct for chemical properties in such triads, it is not the direct explanation for why the atomic mass of the middle element is the average of the other two. Hence, the Reason is true but does not correctly explain the Assertion.
Therefore: Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.

Key Concept: Dobereiner's Triads

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that for the triad lithium (Li), sodium (Na), and potassium (K), the atomic mass of sodium is approximately the average of lithium and potassium.
Step 1: Atomic mass of Li = 7, Na = 23, K = 39.
Step 2: Average of Li and K = $\frac{7 + 39}{2} = \frac{46}{2} = 23$.
Step 3: The atomic mass of Na is exactly equal to this average (23). Thus, the assertion is true.
The reason correctly explains why the assertion holds, as it describes Dobereiner's observation about triads. Hence, both the assertion and reason are true, and the reason is the correct explanation of the assertion.

Key Concept: Periodic Trends, Dobereiner's Triads

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
Dobereiner's Law of Triads states that in a group of three elements with similar chemical properties (triad), the atomic mass of the middle element is roughly the average of the other two. For example, in the triad of Lithium (Li), Sodium (Na), and Potassium (K), the atomic mass of Na ($23$) is close to the average of Li ($7$) and K ($39$): $(7 + 39)/2 = 23$. This assertion is true. However, Dobereiner's classification had limitations as it could not account for all elements and did not predict periodicity based on atomic number, which is the foundation of the modern periodic table. The reason correctly identifies this limitation but does not explain why the assertion about triads is valid. Therefore, both the assertion and reason are true, but the reason does not correctly explain the assertion.

Key Concept: Modern Periodic Table and Dobereiner's Triads

b) Both Assertion and Reason are true, but Reason is NOT the correct explanation of Assertion.
[Solution Description]
The assertion states that Mendeleev's periodic table predicted undiscovered elements because it was based on atomic masses, which is true. Mendeleev left gaps for elements like gallium (eka-aluminum) and germanium (eka-silicon), predicting their properties accurately. The reason mentions Dobereiner's triads, where the middle element's atomic mass was approximately the average of the other two, but this method was limited to a few groups and did not systematically predict new elements. Although both statements are true, Dobereiner's triads were not the basis for Mendeleev's predictions, so the reason does not correctly explain the assertion.

Key Concept: Classification of Elements

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
According to Dobereiner's Triad classification, elements were grouped in sets of three where the atomic mass of the middle element was roughly the average of the other two. For example, Lithium (Li), Sodium (Na), and Potassium (K) form a triad where the atomic mass of Na ($23$) is close to the average of Li ($7$) and K ($39$), which is $\frac{7 + 39}{2} = 23$. Thus, both Assertion and Reason are true, and Reason correctly explains the basis of Dobereiner's classification.

Key Concept: Modern Periodic Law, Electron Configuration

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The Assertion states that elements in the same group have similar chemical properties due to the same number of valence electrons, which is true as valence electrons determine the chemical behavior of an element. The Reason correctly states the modern periodic law formulated by Henry Moseley, which is the basis for the arrangement of elements in the periodic table based on atomic numbers. Since both statements are true and the Reason explains why elements in the same group show periodicity in properties (due to the periodic function of atomic numbers leading to the same valence electron configuration), the correct answer is option a).

Key Concept: Periodic Table Groups

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that sodium Z = 11 and potassium Z = 19 belong to the same group in the modern periodic table, which is true because both are alkali metals (Group 1). The reason given is that they have the same number of valence electrons, which is also correct because both elements have one valence electron in their outermost shell. Moreover, the reason correctly explains why sodium and potassium belong to the same group, as elements with the same number of valence electrons exhibit similar chemical properties and are placed in the same group.

Key Concept: Modern Periodic Table

a) Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
[Solution Description]
The assertion states that the modern periodic table is arranged by atomic number, which is correct as Henry Moseley established that properties depend on atomic number. The reason correctly explains why atomic number is important - because it determines protons/electrons which govern chemical behavior. Both statements are true and the reason properly explains the assertion.

Key Concept: Melting Point, Metallic Character, Compound Formation

c) $M$ is an alkali metal and $N$ is a halogen
[Solution Description]
Step 1: Basic oxides are formed by alkali metals ($M$), which are good conductors of electricity.
Step 2: Diatomic gases forming salts with alkali metals are halogens ($N$). For example, chlorine ($Cl_2$) reacts with sodium to form sodium chloride ($NaCl$).
Step 3: The trend in melting points matches alkali metals (decreasing down Group 1) and halogens (increasing down Group 17).
Conclusion: $M$ is an alkali metal, and $N$ is a halogen.

Key Concept: Electronegativity, Ionisation Energy, Reactivity

c) $A$ forms ionic hydrides and $B$ forms covalent hydrides
[Solution Description]
Step 1: From syllabus, Group 1 consists of alkali metals (lowest $EN$ and $IE$), and Group 17 consists of halogens (highest $EN$ among non-noble gases).
Step 2: Reactivity trend for alkali metals ($A$) increases down the group, while for halogens ($B$), it decreases.
Step 3: Halogens form covalent compounds with hydrogen (e.g., $HCl$), whereas alkali metals form ionic hydrides (e.g., $NaH$).
Conclusion: $A$ forms ionic hydrides, while $B$ forms covalent hydrides.

Key Concept: Atomic Size, Reactivity, Electron Affinity

c) $X$ is an alkali metal and $Y$ is a halogen
[Solution Description]
Step 1: Identify that the element $X$ has the largest atomic size in its period and reacts violently with water. From syllabus, this matches alkali metals like sodium ($Na$) or potassium ($K$).
Step 2: Since $Y$ is a halide and has high electron affinity decreasing down the group, it must be a halogen (e.g., chlorine ($Cl$) or fluorine ($F$)).
Step 3: Alkali metals ($X$) are highly reactive reducing agents, while halogens ($Y$) are strong oxidising agents.
Conclusion: $X$ is an alkali metal, and $Y$ is a halogen.

Key Concept: Physical States

c) Bromine ($Br$)
[Solution Description]
According to the syllabus, bromine ($Br_2$) is described as a poisonous red-brown volatile liquid, whereas chlorine ($Cl_2$) and fluorine ($F_2$) are gases, and iodine ($I_2$) is a dark grey crystalline solid.

Key Concept: Reactivity and Nature

c) Reactivity increases down the group for alkali metals but decreases for halogens.
[Solution Description]
Alkali metals are highly reactive metals whose reactivity increases down the group, while halogens are highly reactive non-metals whose reactivity decreases down the group. This is due to differences in electronegativity and valence electrons, as outlined in the syllabus.

Key Concept: Electron Affinity

c) Electron affinity decreases down the group.
[Solution Description]
The syllabus states that halogens have high electron affinity values, which decrease down the group. This is because as the atomic size increases, the tendency to attract additional electrons decreases due to increased distance from the nucleus.

Key Concept: Reaction with Water

c) Liberate hydrogen gas
[Solution Description]
Alkali metals react vigorously with water to produce hydrogen gas and an alkaline solution (e.g., $2Na + 2H_2O \rightarrow 2NaOH + H_2$). They do not form covalent bonds but release hydrogen.

Key Concept: Conduction

c) Non-conductors of electricity
[Solution Description]
Halogens are non-metals and do not conduct electricity, unlike alkali metals which are good conductors.

Key Concept: Valence Electrons

a) One
[Solution Description]
Alkali metals belong to Group 1 and have one electron in their outermost shell. Thus, they possess one valence electron.

Key Concept: Mass number, Odd proton count

c) The neutron count is 12, consistent with the $2Z + 1$ rule and stability.
[Solution Description]
The atomic number is 11 (odd), so according to the syllabus, the mass number should generally be $2Z + 1 = 2 \times 11 + 1 = 23$. This matches the given mass number. The number of neutrons is $23 - 11 = 12$, which satisfies the condition for stability ($n/p \approx 1$).

Key Concept: Mass number, n/p ratio

c) The neutron/proton ratio is approximately 1, making it stable.
[Solution Description]
The atomic number is 12, so the number of protons is 12. The mass number is 24, so the number of neutrons is $24 - 12 = 12$. The neutron/proton ratio is $12/12 = 1$, which matches the stability condition mentioned in the syllabus for light elements.

Key Concept: Mass number, Atomic number

c) 18
[Solution Description]
The given atomic number is 8, so the number of protons is 8. For the first isotope, mass number is 16, so the number of neutrons is $16 - 8 = 8$. The second isotope has 2 more neutrons, so it has $8 + 2 = 10$ neutrons. Therefore, the mass number of the second isotope is $8 + 10 = 18$.

Key Concept: Even-Odd Proton Rule

b) 19
[Solution Description]
According to the even-odd proton rule, for elements with an odd number of protons ($Z$), the mass number ($A$) is generally given by $A = 2Z + 1$.
Given:
Atomic number ($Z$) = 9
Applying the formula:
$A = 2 \times 9 + 1 = 19$
Therefore, the expected mass number is 19.