Question

Which one of the following ionic species has the greatest proton affinity to form stable compound?

• A. $H{S}^{-}$
• B. $N{H}_2^{-}$
• C. $F^{-}$
• D. $I^{-}$

Answer 1

Answer: (B)

Proton affinity decreases across a period, from left to right, and down a group, from top to bottom. Thus, nitrogen family has the highest proton affinity, followed by oxygen family, followed by halogens. Also, the ionic species with the greatest proton affinity forms the strongest base or the weakest acid. Of all the acids that these ionic species will form $(HI,H_{2}S,N{H}_3$ and $HF$), $N{H}_3$ is the weakest acid. Hence, $N{H}_2^{-}$ has the greatest proton affinity.