7.19 Knowing the electron gain enthalpy values for O → O and O →O2 as –141 and 702 kJ  mol–1 respectively, how can you account for the formation of a large number of oxides having O2- species and not O-?

(Hint: Consider the lattice energy factor in the formation of compounds).

Stability of an ionic compound depends on its lattice energy. More the lattice energy of a compound, more stable it will be. Lattice energy is directly proportional to the charge carried by an ion. When a metal combines with oxygen, the lattice energy of the oxide involving O2 ion is much more than the oxide involving O ion. Hence, the oxide having O2 ions are more stable than oxides having O. Hence, we can say that the formation of O2 is energetically more favourable than the formation of O.