Question

Which one of the following substances has the highest proton affinity?

• A. $H_{2}O$
• B. $H_{2}S$
• C. $N{H}_3$
• D. $P{H}_3$

Answer 1

The correct option is C $N{H}_3$

Ammonia acts as a lewis base, accepting proton to form ammonium ion as it has tendency to donate an electron pair.
$H-\stackrel{\stackrel{H}{|}}{\underset{\underset{H}{|}}{N:}}+H^{+}⟶$ ${\left[\begin{array}{c}H-\stackrel{\stackrel{H}{|}}{\underset{\underset{H}{|}}{N}}\to H\end{array}\right]}^{+}$
The conjugate acid $\left[N{H}_4^{+}\right]$ is also stable.
Moreover the electron density of $N$ atom of $N{H}_3$ is also high, on the other hand $P$ and $S$ possesses empty d- orbitals and electronegativity of $O$ atom is high compared to $N$ atom.