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–.