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 .