The formation of oxide ion $\mathrm{O}^{2-}(g)$ requires first an exothermic and then an endothermic step as shown below: $\mathrm{O}(g) + e^- \longrightarrow \mathrm{O}^-(g); \Delta H^\circ = -142\mathrm{kJ}\mathrm{mol}^{-1}$ and $\mathrm{O}^-(g) + e^- \longrightarrow \mathrm{O}^{2-}(g); \Delta H^\circ = 844\mathrm{kJ}\mathrm{mol}^{-1}$. This is because