Question

Consider the elements $N,P,O,S$ and arrange them in order of
(a) increasing first ionization enthalpy,
(b) increasing negative electron gain enthalpy,
(c) increasing non-metallic character.

Answer 1

Arranging all the elements into different groups and periods in order of their increasing atomic numbers, we have,
(a) Since ${\Delta }_i{H}_1$ decreases down a group, therefore, ${\Delta }_i{H}_1$ of $N$ and $O$ are higher than those of $P$ and $S$. Further since $N$ has more stable exactly half-filled electronic configuration in the 2p-subshell, therefore, it is more difficult to remove out an electron from $N$ than from $O$ even though $O$ has higher nuclear charge. Similarly, $P$ has exactly half-filled electronic configuration in the 3p-subshell. Therefore, ${\Delta }_i{H}_1$ of $P$ is higher than that of $S$. Thus, the overall increasing order of first ionization enthalpy of these elements follows the order: $S<P<0<N$.
(b) Since adding an electron to smaller size 2p-orbital causes greater repulsion than adding an electron to larger 3p-orbital, therefore, electron gain enthalpies of $P$ and $S$ are more negative than those of $N$ and $O$ respectively. Further since $S$ has higher nuclear charge but $P$ has more stable exactly half-filled electronic configuration in the 3p-subshell, therefore, it is easier to add an electron to $S$ than to $P$. In other words, electron gain enthalpy of $S$ is more negative than that of $P$. Similarly, $N$ has exactly half-filled electronic configuration in the 2p-subshell but $O$ has higher nuclear charge.
But the addition of an electron to $N$ causes repulsions to such an extent that electron gain enthalpy of $N$ is actually positive while that of $O$ as expected in negative. Combining all the above results, the increasing order of negative electron gain enthalpy of these elements follows the order: $N<P<0<S$.
(c) Since non-metallic character decreases down a group but increases along a period, therefore, $O$ is the most non-metallic element while $P$ is the least non-metallic element. The actual order of increasing non-metallic character is: $P<S<N<O$.