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Which one of the following ionic species has the greatest proton affinity to form stable compound?
A
$NH_2^-$
B
$F^-$
C
$I^-$
D
$HS^-$
Detailed Solution
The proton affinity of an anion is a measure of its basic strength: the stronger the base, the greater its tendency to accept a proton.
A stronger base is the conjugate base of a weaker acid.
The conjugate acids of the given ions are $NH_3$, $HF$, $HI$ and $H_2S$.
Acid strength: $HI \gt HF \gt H_2S \gt NH_3$. Ammonia is by far the weakest acid among these.
So $NH_2^-$ is the strongest base and has the greatest proton affinity, forming the stable compound $NH_3$.
In general, proton affinity decreases on moving across a period from left to right, and within a group it decreases from top to bottom: nitrogen family > oxygen family > halogens.
A stronger base is the conjugate base of a weaker acid.
The conjugate acids of the given ions are $NH_3$, $HF$, $HI$ and $H_2S$.
Acid strength: $HI \gt HF \gt H_2S \gt NH_3$. Ammonia is by far the weakest acid among these.
So $NH_2^-$ is the strongest base and has the greatest proton affinity, forming the stable compound $NH_3$.
In general, proton affinity decreases on moving across a period from left to right, and within a group it decreases from top to bottom: nitrogen family > oxygen family > halogens.