The first ionisation enthalpy vlaues of third period element, Na, Mg and Si are respectively 496,737 and 786 kJ ${\mathrm{mol}}^{-1}$. The first ${\mathrm{\Delta }}_{\mathrm{i}}\mathrm{H}$ value for Al will be close to

It will be more close to 575 kJ ${\mathrm{mol}}^{-1}$. The value for Al should be lower than that of Mg because of effective shielding of 3p-electrons from the nucleus by 3s- electrons.