On going down a group the ionization enthalpies generally decrease because the size of the atom increases and screening effect of the inner shell electrons which is dominates or masks the effect of increase in nuclear charge.

Consequently, the electron becomes weakly held by the nucleus as we move down the group. The sharp decrease in ionization enthalpy from boron to aluminium is due to increase in the size of the atom. But the Gallium atom contains 10 electrons in the d subshell. The shielding tendency of d electrons is less as compared to the electrons of s and p subshell, the outer valence electrons are tightly held to the nucleus. This is the reason why the ionization enthalpy increases slightly on moving down the group from Al to Ga in spite of increase in their atomic size. The similar effect is also observed from In to Tl. In case of tellurium 14f electrons provide very low shielding effect.