The formation of the oxide ion O²⁻(g), from oxygen atom requires first an exothermic and then an endothermic step as shown below: O(g) + e⁻ → O⁻(g); ΔegH = −141 kJ mol⁻¹ O⁻(g) + e⁻ → O²⁻(g); ΔegH = +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