Chemical Bonding and Molecular Structure Part 2

Assertion (A):This statement is taken directly from NCERT (group 15, nitrogen family).From $N$ to $P$ a completely new shell is added, so the covalent radius increases considerably, from about $70\ \text{pm}$ to about $110\ \text{pm}$.From $As$ to $Bi$ the covalent radius also increases, but only slightly, from about $120\ \text{pm}$ to about $150\ \text{pm}$.Hence (A) is a true statement.Reason (R):Down a group the number of shells increases, so the covalent and ionic radii in a particular oxidation state do increase down the group.Hence (R) is also a true statement.Is (R) the correct explanation of (A)?No.(R) only states the general downward trend and does not explain why the increase becomes only marginal from $As$ to $Bi$.The actual cause of the small increase in covalent radius from $As$ to $Bi$ is the presence of completely filled $d$- and $f$-orbitals in the heavier members of the group.Electrons in these inner $d$ and $f$ orbitals shield the valence electrons poorly, so the effective nuclear charge increases ($Z_{\text{eff}}$ increases) and the atom contracts, largely offsetting the expected increase in size.Conclusion:Both (A) and (R) are true, but (R) is not the correct explanation of (A).The correct option is: Both (A) and (R) are true but (R) is not the correct explanation of (A).