The formation of 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
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Although 
has noble gas configuration as it is isoelectronic with Neon but its formation is unfavourable due to the strong electronic repulsion between the negatively charged 
ion and the second electron being added. Hence, the electron repulsion outweighs the stability gained by achieving noble gas configuration.
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