Electronic configuration of 13Al = 2,8, 3
= 1s2 , 2s2 , 2p6 , 3s2 , 3p1
Electronic configuration of 31Ga = 2, 8, 18, 3
= 1s2 , 2s2 , 2p6 , 3s2 , 3p6 , 3d10 , 4s2 , 4p1
In Ga, 10 electrons present in 3d- sub shell do not shield the outer electrons from the nucleus effectively. As a result, effective nuclear charge in Ga increases. So, the ionization enthalpy of Ga is slightly higher than AL.