The formation of oxide ion `O^(2-)(g)` from oxygen atom requires first an exothermic and then an endothermic step as shown below
`O(g)+e^(-) rarr O^(-)(g), DeltaH^(-) = - 141 kj mol^(-1)`
`O^(-)(g) +e^(-) rarr O^(2-) (g), DeltaH^(-) =+ 780 kj mol^(-1)`
Thus, process of formation of `O^(2-)` in gas phase is unfavourable even through `O^(2-)` is isoelectronic with neon. It is due to the fact that