question_answer

The formation of the oxide ion \[{{O}^{2-}}(g)\], from oxygen atom requires first an exothermic and then an endothermic step as shown below, \[O(g)+{{e}^{-}}\to {{O}^{-}}(g);\Delta f{{H}^{{}^\circ }}=-141KJmo{{l}^{-1}}\] \[{{O}^{-}}(g)+{{e}^{-}}\xrightarrow{\,}\,{{O}^{2-}}\,(g);\,{{\Delta }_{f}}{{H}^{\text{o}}}\] \[=+780\,kJm\,o{{l}^{-1}}\] Thus, process of formation of \[{{O}^{2-}}\] in gas phase is unfavourable even though \[{{O}^{2-}}\] is isoelectronic with neon. It is due to the fact that

A)  electron repulsion outweighs the stability gained by achieving noble gas configuration

B)  \[{{O}^{-}}\] ion has comparatively smaller size than oxygen atom

C)  Oxygen is more electronegative

D)  addition of electron in oxygen result in large size of the ion