Question
Give reason:
$NH_3$ has a higher proton affinity than $PH_3.$

Answer

When $NH_3$ or $PH_3$ accepts a proton, an additional $N-H$ or $P-H$ bond is formed.
$H_3N : + H^+ → NH + 4$
$H^3P : + H^+ → PH + 4$
Due to the bigger size of $P$ than $N$, $P-H$ bond thus formed is much weaker than the $N-H$ bond. Thus, $NH_3$ has higher proton affinity than $PH_3.$

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