Why `MgO` exist as `Mg^(2+)O^(2-)` not as `Mg^(o+)O^(ɵ)` whereas the formation of `Mg^(2+)` from `Mg` requires more energy than formation of `Mg^(o+)` and formation of `O^(ɵ)` from `O` is exothermic whereas the formation of `O^(2-)` is endothermic .

Explanation

`MgO` exists in crystalline state.
Greater the charge density `(("Charge")/("size")"ratio")` on ions the greater is release of lattice energy. So in `MgO, Mg` exists as `Mg^(2+)` and `O` exists as `O^(2-)`