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

 endothermic step as shown below: 

$O\left(g\right)+e^{-}\to O^{-}\left(g\right);{\Delta }_{eg}H=-141kJmo{l}^{-1}$

$O^{-}\left(g\right)+e^{-}\to O^{2-}\left(g\right);{\Delta }_{eg}H=+780kJmo{l}^{-1}$

Thus process of formation of  $O^{2-}$ in gas phase is unfavorable even $O^{2-}$ is isoelectronic with neon. It is due to the fact that: