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Solution


Q.

The formation of the oxide ion, O(g)2- from oxygen atom requires first an exothermic and then an endothermic step as shown below:

O(g)+e-→O(g)-;  ΔfH°=-141 kJ mol-1

O(g)-+e-→O(g)2-;  ΔfH°=+780 kJ mol-1

Thus, process of formation of O2- in gas phase is unfavourable even though O2- is isoelectronic with neon. It is due to the fact that,     [2015]
1 O- ion has comparatively smaller size than oxygen atom  
2 oxygen is more electronegative  
3 addition of electron in oxygen results in larger size of the ion  
4 electron repulsion outweighs the stability gained by achieving noble gas configuration.  

Ans.

(4)