Which of the following ions is most unlikely to exist?

The process requiring the absorption of energy is

Nitrogen has lower electron affinity than its preceding element carbon because

The difference between electron gain enthalpies will be maximum between:

Arrange $N$,$O$,and $S$ in order of decreasing electron affinity.

The formation of the oxide ion,$O^{2-}_{(g)}$ from an oxygen atom requires first an exothermic and then an endothermic step as shown below:
$O_{(g)} + e^- \to O^{-}_{(g)} ; \Delta_f H^o = -141 \ kJ \ mol^{-1}$
$O^{-}_{(g)} + e^- \to O^{2-}_{(g)} ; \Delta_f H^o = +780 \ kJ \ mol^{-1}$
Thus,the process of formation of $O^{2-}$ in the gas phase is unfavourable even though $O^{2-}$ is isoelectronic with neon. This is due to the fact that,