The formation of oxide ion O2− g, from oxygen atom requires first an exothermic and then an endothermic step as shown belowO(g)+e−→O−(g);ΔH⊖=−141kJmol−1O−(g)+e−→O−2(g);ΔH⊖=+780kmol−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