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JEE Chemistry 2024 Question with Solution

Given below are the two statements: one is labeled as Assertion (A) and the other is labeled as Reason (R). Assertion (A): There is a considerable increase in covalent radius from N to P. However, from As to Bi only a small increase in covalent radius is observed. Reason (R): Covalent and ionic radii in a particular oxidation state increase down the group. Choose the most appropriate answer from the options below:

  • A

    (A) is false but (R) is true

  • B

    Both (A) and (R) are true but (R) is not the correct explanation of (A)

  • C

    (A) is true but (R) is false

  • D

    Both (A) and (R) are true and (R) is the correct explanation of (A)

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: An assertion-reason question about periodic trends in covalent radius down a group.

Find: Which option correctly describes the truth of Assertion (A) and Reason (R), and whether (R) explains (A).

The solution states that the covalent radius generally increases down a group because additional electron shells are added. Therefore, the statement in (R) is true.

For Assertion (A), the increase from N to P is considerable, but from As to Bi only a small increase is observed. The solution attributes this smaller increase to the effect of filled d and f orbitals and their poor shielding, so (A) is also true.

Since (R) only gives the general trend that radii increase down the group, it does not specifically explain why the increase from As to Bi is small.

Therefore, both (A) and (R) are true, but (R) is not the correct explanation of (A). The correct option is B.

Detailed Explanation

Given:

  • Assertion (A): There is a considerable increase in covalent radius from N to P. However, from As to Bi only a small increase in covalent radius is observed.
  • Reason (R): Covalent and ionic radii in a particular oxidation state increase down the group.

Find: The correct relationship between (A) and (R).

  1. The solution first notes that atomic size and covalent radius usually increase down a group because new electron shells are added.
  2. Hence, the statement in (R) is true as a general periodic trend.
  3. The solution then evaluates (A) and states that the increase from N to P is significant.
  4. It further states that from As to Bi, the increase is comparatively small because filled d- and f-orbitals lead to poor shielding effects.
  5. So Assertion (A) is also true.
  6. However, Reason (R) only states the general increase in radii down the group. It does not explain the exceptional feature that the increase from As to Bi is only small.

Thus, both statements are true, but the reason is not the correct explanation of the assertion.

The correct option is B.

Common mistakes

  • Assuming that a true reason must always explain a true assertion. In assertion-reason questions, both statements can be true independently, yet the reason may still fail to explain the specific observation. Always test the explanatory link separately.

  • Using only the general trend that radius increases down a group and ignoring the exceptional small increase from As to Bi. The key point here is not just increase, but why the increase becomes smaller. Consider shielding effects as stated in the solution.

  • Marking the option that both statements are true and the reason explains the assertion. This is incorrect because the reason mentions only the overall increase in radii, not the cause of the reduced increase in the lower elements of the group.

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