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
ion has comparatively smaller size than oxygen atom. Text Solution
Verified by ExpertsA
It is due to the fact that electron repulsion exceeds the stability gained by achieving noble gas configuration. When an electron is added to 
anion, there is strong electrostatic repulsion between the negative charges. Due to this, the second electron gain enthalpy of oxygen is positive.
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