The formation of oxide ion ${\mathrm{O}}^{2-}\left(\mathrm{g}\right),$ from oxygen atom requires first an exothermic and then an endothermic step as shown below

$\begin{array}{l}\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\to {\mathrm{O}}^{-}\left(\mathrm{g}\right);{\mathrm{\Delta H}}^{\ominus }=-141{\mathrm{kJmol}}^{-1}\\ {\mathrm{O}}^{-}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\to {\mathrm{O}}^{-2}\left(\mathrm{g}\right);{\mathrm{\Delta H}}^{\ominus }=+780{\mathrm{kmol}}^{-1}\end{array}$

Thus, process of formation of ${\mathrm{O}}^{2-}$ in gas phase is unfavourable even though ${\mathrm{O}}^{2-}$ is isoelectronic with neon. It is due to the fact that