The formation of the oxide ion `O^(2-)(g)`, form oxygen atom requires first an exothermic and then an endothermic step as shown below,
`O(g)+e^(-) to O^(-)(g),DeltaH_(f)^(0)=-141kJ*mol^(-1)`
`O^(-)(g)+e^(-) to O^(2-)(g),DeltaH_(f)^(0)=+780kJ*mol^(-1)` ltbr. Thus, process of formations of `O^(2-)` in gas phase is unfavourable even through `O^(2-)` is isoelectronic with neon. it is due to the fact that-

Options

A. electron repulsion outweighs the stability gained by achieving noble gas configuration
B. `O^(-)` ion has comparatively smaller size than O-atom
C. oxygen is more electronegative
D. addition of electron in O result in larger size of the ion

Answer

A

Explanation

Electron repulsion predominates over the stability gained by achieving noble gas configuration. Hence, formation of `O^(2-)` in gas phase is unfavourable.