Second ionisation enthalpy of Mg is sufficiently high and second electron-gain enthalpy of O has a positive value. How do you explain the existence of `Mg^(2+)O^(2-)` rather than `Mg^(+)O^(-)`?

Explanation

Lattice energy of an ionic crystal depends on the force of attraction between the cations and anions `(F prop (q_(1)q_(2))/(r^(2)))`
So, magnitude of lattice energy increases as the charges on the cation and anion increases. Consequently, the lattice energy of `Mg^(2+)O^(2-)` is very much greater than that of `Mg^(+)O^(-)` in fact the lattice energy of `Mg^(2+)O^(2-)` is so high that it exceeds the unfavourable effects of second ionisatttopy of Mg and the second electron-gain enthalpy of O. so, `Mg^(2+) O^(2-)` is a stable ionic compound and its formation is favoured over `Mg^(+)O^(-)`.