Na configuration: 1s2,2s2,2p6,3s1.
Mg configuration: 1s2,2s2,2p6,3s2.
Al configuration: 1s2,2s2,2p6,3s2,3p1
Si configuration: 1s2,2s2,2p6,3s2,3p2
As effective nuclear charge on Na is less than Mg, so the ionisation enthalpy of Na will be less than Mg as Mg has 2 electrons and Na has one electron in the outermost shell.
It is easier to remove an electron from p orbital than in s orbital so the ionisation enthalpy of Mg is less than Al. Therefore, Ionisation enthalpy of Mg is greater than Al.
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