The formation of the oxide ion `O^(2-)`(g), from oxygen atom requires first an exothermic and then an endothermic step as shown below :
`O(g)+e^(-) to O(g): Delta_(f)H^(Ѳ)= - 141 "kJ mol"^(-1)`
`O^(-)(g)+e^(-) to O^(2-) (g), Delta_(f)H^(Ѳ)= + 780 "kJ mol"^(-1)`
Thus process of formation of `O^(2-)` in gas phase in unfavourable even though `O^(2-)` is isoelectronic with neon. It is due to the fact that

Options

A. Electron repulsion out weigths the stability gained by achieving noble gas configuration
B. `O^(-)` ion has comparatively smaller size than oxygen atom
C. Oxygen is more electron negative
D. Addition of electron in oxygen results in larger size of the ion.

Answer

A

Explanation

Incoming electrons occupies the smaller n=2 shell, also negative charge on oxygen `(O^(-))` is another factor due to which incoming electron feel requlsion. Hence electron repulsion outweigh the stability gained by achieving noble gas configuration.