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

${\mathrm{O}}_{\left(\mathrm{g}\right)}+{\mathrm{e}}^{-}\to {\mathrm{O}}_{\left(\mathrm{g}\right)}^{-};{\mathrm{\Delta }}_{\mathrm{f}}\mathrm{H}°=-141 \mathrm{kJ}{ \mathrm{mol}}^{-1}$

${\mathrm{O}}_{\left(g\right)}^{-}+{\mathrm{e}}^{-}\to {\mathrm{O}}_{\left(\mathrm{g}\right)}^{2-};{\mathrm{\Delta }}_{\mathrm{f}}\mathrm{H}°=+780 \mathrm{kJ}{ \mathrm{mol}}^{-1}$

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