The discontinuity in the ionisation enthalpy values observed between

The ionisation enthalpy values as expected from the general trends, do not decrease smoothly down the group. The decrease from B to Al is associated with increase in size. The observed discontinuity in the ionisation enthalpy values between Al and Ga, and between In and Tl are due to the inability of d-and f-electrons, which have low screening effect, to compensate the increase in nuclear charge.