In the third period, Al comes after Mg and before Si. Since in Mg (1s22s22p63s2 ), the last electron is to be lost from a fully filled 3s-orbital while in Al (ls22s22p63s23p1) it is to be lost from a 3p-orbital, therefore, ΔiH1 of Al is lower than that of Mg (737kJmol−1). Further, nuclear charge increases in going from Na to Al, therefore ΔiH1 of Al is higher than that of Na(496kJmol−1) but lower than that of Mg (737kJmol−1). Therefore, the appropriate value of ionisation potential of Al is 577kJmol−1.