Mg has relatively higher ionisation enthalpy than Al although the atomic number of the latter is more than the former-explain why?

Explanation

Electronic configuration of `""_(12)Mg:1s^(2)2s^(2)2p^(6)3s^(2)`
Electronic configuration of `""_(13)Al:1s^(2)2s^(2)2p^(6)3s^(2)3p^(1)`
Penetration effect of s-electron is greater than that of p-electron. So it is easier to remove the 3p-electron from the outermost shell of Al. furthermore, the removal of this electron gives `Al^(+)`, which has fully filled 3s-orbital (stable electronic configuration) in the outermost shell. on the other hand Mg, atom has fully filled 3s-orbital (stable electronic configuration) in its ground state. removal of one electron from 3s-orbital of Mg-atom will required large amount of energy because the resulting `Mg^(+)` ion will have less stable electronic configuration `(1s^(2)2s^(2)2p^(6)3s^(1))`. Furthermore, it is rather difficult to remove an electron fromo s-orbital having greater penetration effect. so ionisation enthalpy of Mg is greater than that of Al.