The first first ionization potential of Na, Mg, Al and Si are in the order

IE1 of Mg is higher than that of Na because of increased nuclear charge and also that of Al because in Mg a 3s-electron has to be removed while in Al it is the 3p-electron. The IE1 of Si is, however, higher than those of Mg and Al because of its increased nuclear charge. Thus, the overall order is Na < Mg > Al < Si .