Question

Knowing the electron gain enthalpy values for $O\to O^{-}$ and $O\to O^{2-}$ as $-141$ and $720kJmo{l}^{-1}$ respectively , how can you account for the formation of a large number of oxides having $O^{2-}$ 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 $O^{2-}$ ion is much more than the oxide involving $O^{-}$ ion. Hence, we can say that formation of $O^{2-}$ is energetically more favourable than formation of $O^{-}$.