The process of formation of O2− in the gas phase is unfavorable even through O2− is isoelectronic with neon. It is due to that electronic repulsion outweighs the stability gained by achieving noble gas configuration. The formation of oxide ion, O2−(g) from oxygen atom require first an exothermic and then endothermic step. O(g)+e−→O−(g);ΔH⊝=−141kJmol−1 O−(g)+e−→O2−(g);ΔH⊝=+760kJmol−1 When an electron is added to O−anion, there is strong electrostatic repulsion between two negative charge due to this, the second electron gain enthalpy of oxygen is positive.