The formation of the oxide ion, , 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,
(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 noble gas configuration.
(iv) O– ion has comparatively smaller size than oxygen atom.
The answer is the option (iii) electron repulsion outweighs the stability gained by achieving noble gas configuration.
Explanation: Energy is released whenever an electron is added to the O atom to form O- ion. Thus the first electron gain enthalpy of O is negative. Contrary to this whenever an electron is added to O- ion to form O2- ion, energy has to be given out in order to overcome the very strong electronic repulsions, and that is why the second gain enthalpy of O is positive.