The formation of the oxide ion O2-(g), from oxygen atom requires first an exothermic and then an endothermic step as shown below:
Thus process of formation of in gas phase is unfavourable even though is isoelectronic with neon. It is due to the fact that
Electron repulsion outweighs the stability gained by achieving noble gas configuration
ion has comparatively smaller size than oxygen atom.
Oxygen is more electronegative.
Addition of electron in oxygen results in larger size of the ion.