The formation of the oxide ion,

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) ; ; ∆ HV = – 141 kJ mol–1

O– (g) + e– → O2– (g) ; ∆ HV = + 780 kJ mol–1

Thus the process of formation of O2– in the gas phase is unfavourable even though

O2– is isoelectronic with neon. It is due to the fact that

(i) oxygen is more electronegative.

(ii) addition of electron in oxygen results in larger size of the ion.

(iii) electron repulsion outweighs the stability gained by achieving a noble

gas configuration.

(iv) O–

ion has a comparatively smaller size than an oxygen atom.