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) ; ∆ H ς = – 141 kJ mol–1 O– (g) + e–  → O2– (g); ∆ H ς = + 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,

Question

Philoid · Accepted Answer

Ois an electronegative element so when we add an electron to it, it gains that electron and forms O^- anion. Thus energy is released in the formation and it is an exothermic process. But while another electron is added to this unstable O^- anion then it faces repulsion due to the similar charges. So in order to overcome these repulsions due to similar charge external energy is required due to which this becomes an endothermic process.