On moving down the group from B to Tl, a regular decreasing trend in the ionisation energy values is not observed.

This is due to Ga, there are 10 d-electrons in the penultimate shell which screen the nuclear charge less effectively.

Thus, outer electronsare held firmly. As a result, the ionization energy of both Al and Ga is nearly the same.

The increase in ionization energy from In to Tl is due to poor screening effect of 14f electrons present in the inner shell.