Magnesium (Mg) has the first ionisation enthalpy higher than that of Al. Due to fulfilled s-orbital of Mg it requires higher energy to remove the electron from outermost shell. On the other hand, Al can easily lose one electron as its p-orbital has only one electron. ∴IE1(Mg)=737.7‌kJ‌mol−1, IE1(Al)=577.5‌kJ‌mol−1