The formation of the oxide ion, O2- (g), from oxygen atom requires first an exothermic and then an endothermic step as shown below:

    \[\mathbf{O}\text{ }\left( \mathbf{g} \right)\text{ }+\text{ }\mathbf{e}\text{ }\to \text{ }{{\mathbf{O}}^{-}}\left( \mathbf{g} \right)\text{ };\text{ };\text{ }\text{ }\mathbf{HV}\text{ }=\text{ }\text{ }\mathbf{141}\text{ }\mathbf{kJ}\text{ }\mathbf{mol}\mathbf{1}\]


    \[{{\mathbf{O}}^{-}}\left( \mathbf{g} \right)\text{ }+\text{ }\mathbf{e}\text{ }\to \text{ }{{\mathbf{O}}^{\mathbf{2}-}}\left( \mathbf{g} \right)\text{ };\text{ }\text{ }\mathbf{HV}\text{ }=\text{ }+\text{ }\mathbf{780}\text{ }\mathbf{kJ}\text{ }\mathbf{mol}\mathbf{1}\]


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.
The formation of the oxide ion, O2- (g), from oxygen atom requires first an exothermic and then an endothermic step as shown below:

    \[\mathbf{O}\text{ }\left( \mathbf{g} \right)\text{ }+\text{ }\mathbf{e}\text{ }\to \text{ }{{\mathbf{O}}^{-}}\left( \mathbf{g} \right)\text{ };\text{ };\text{ }\text{ }\mathbf{HV}\text{ }=\text{ }\text{ }\mathbf{141}\text{ }\mathbf{kJ}\text{ }\mathbf{mol}\mathbf{1}\]


    \[{{\mathbf{O}}^{-}}\left( \mathbf{g} \right)\text{ }+\text{ }\mathbf{e}\text{ }\to \text{ }{{\mathbf{O}}^{\mathbf{2}-}}\left( \mathbf{g} \right)\text{ };\text{ }\text{ }\mathbf{HV}\text{ }=\text{ }+\text{ }\mathbf{780}\text{ }\mathbf{kJ}\text{ }\mathbf{mol}\mathbf{1}\]


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.

Option (iii) is the answer. This is due to the fact that when an electron is introduced to a negatively charged ion, it is repelled rather than attracted.

As a result, the addition of the second electron normally necessitates the use of energy. As a result, the second electron affinity values are positive, indicating that the system is endothermic.