Question

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 lattice energy factor in the formation of compounds).

Answer 1

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 formation of O²⁻ is energetically more favourable than formation of O−.