The formation of the oxide ion, O^2-( g ), from oxygen atom requires first an exothermic and then an endothermic step as shown below: O ( g )+ e^- →…

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