Saurabh Koranglekar
On moving down a group the ionization enthalpies generally decreses due to increase in atomic size & screening effect which is more than to compensate the effect of increase in nuclear charge.Consequently the electron becomes less & less tightly held by the nucleus as we move down the group.The sharpe decrease in ionization enthalpy from B to Al is due to increase in size.In case of Ga ,there are 10 d electrons in its inner electronic configuration..since the d electrons shield the nuclear charge less effectively than the s & p electrons,the outer electron are held strongly by the nucleus.As a result the ionization enthalpy increases slightly inspite of the increase in atomic size as we move from Al to Ga. The similar increase is also observed from In to Tl, which is due to presence of 14f electrons in inner electronic configuration of Tl which has poor shielding effect.
