Question

Which of the following ion has the largest radius?

• A. $C{l}^{-}$
• B. $S^{2-}$
• C. $N{a}^{+}$
• D. $F^{-}$

Answer 1

Answer: (B)

$S^{2-}$

Atomic radius increase down a group as the number of electrons increases.

For isoelectronic species, anions are larger than the neutral atom which in turn is larger than the cations.

Consider all the options:-
A) $C{l}^{-}\to$ Anion with 18$e^{-}$.

B) $S^{2-}\to$ Anion with 18$e^{-}$.

C) $N{a}^{+}\to$ Cation with 10$e^{-}$.

D) $F^{-}\to$ Anion with 10$e^{-}$.

As reasoned above, species with 18$e^{-}$ are larger than species with 10$e^{-}$. Hence, the largest species is either $C{l}^{-}$ or $S^{2-}$. Since in the case of $S^{2-}$ there will be greater electron-electron repulsion, $S^{2-}$ will be larger than $C{l}^{-}$.

Hence, option B is the right answer.