The formation of oxide ion O2-(g), from oxygen atom requires first an exothermic and then an endothermic step as shown below.O(g)+e-⟶O-(g);ΔH-=-141 kJ mol-1O-(g)+e-⟶O2( g);ΔH-=+780 kJ mol-1Thus, process of formation of O2- in gas phase is unfavourable even though O2- - is isoelectronic with neon. It is due to the fact that