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

Nuclear charge rises, atomic size falls, and ionisation enthalpy rises as we travel through the era. $\mathrm{Al}\left(3{\mathrm{s}}^{2}3{\mathrm{p}}^1\right)$ requires the removal of an electron from a partially filled 3p orbital, whereas Mg (3s 2) requires the removal of an electron from a stable, fully filled 3s orbital. More energy is needed to remove an electron from a stable, fully filled orbital than from a partially filled orbital. As a result, the ionisation enthalpy for magnesium is higher than that of aluminium. Therefore, Na>Mg>Al>Si is the proper sequence of the first ionisation enthalpies.