The formation of the oxide ion, O2−(g) from oxygen atom requires first an exothermic and then an endothermic step as shown below:
O(g)+e−→O−(g);ΔfH∘=−141 kJ mol−1O−(g)+e−→O2−(g);ΔfH∘=+780 kJ mol−1
Thus, process of formation of O2− in gas phase is unfavourable even though O2− is isoelectronic with neon. It is due to the fact that,
electron repulsion outweighs the stability gained by achieving noble gas configuration.
When the oxygen atom gains one electron, it makes it more stable, which is reflected by the first equation. Adding another electron is really difficult due to the repulsion faced by it. This is shown by the second equation.