Given below are two statements : one is labelled as Assertion (A) and the other is labelled 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 increases down the group. In the light of the above statements, choose the most appropriate answer from the options given below:

Solution

Chemistry ma’am · 4 steps

Both statements are true, but R only says radii grow down the group. It never says why the growth slows.

1. Check the numbers. Covalent radii in group 15 are N 70 pm, P 110 pm, As 121 pm, Sb 141 pm and Bi 148 pm. N to P is a jump of 40 pm in a single step. As to Bi spans two steps and adds only 27 pm. So (A) is true.
2. Check the reason. Each step down the group adds a new shell, so covalent radii and ionic radii in the same oxidation state do increase. So (R) is also true.
3. Real cause. From As onwards, the atoms have filled 3d, 4d or 4f subshells. These d and f electrons shield the nucleus poorly, so the effective nuclear charge rises and pulls the outer shell in. That is why the increase from As to Bi is small.
4. So. (R) explains why radii increase but not why the increase becomes small, and that small increase is the point of (A). Both are true, but (R) is not the correct explanation, so the answer is (c).

The idea to keep. In assertion-reason questions, ask whether R explains the specific part A stresses. A small increase down a group after period 3 comes from poor shielding by filled d and f orbitals.

Answer from the NTA final answer key. Solution by PrepWiser.