On moving from B to Al, all the ionisation enthalpies decreases as expected and this decrease is due to an increase in atomic size and shielding effect.
On moving from Al to Ga, the ionisation enthalpy increases slightly, because on moving from Al to Ga, both nuclear charge and shielding effect increase but due to poor shielding by d-electron in Ga, effective nuclear charge on valence electron increases resulting in d-block contraction, that is why ionisation enthalpies increase.
On moving from Ga to In, again there is slight decrease in ionisation enthalpies due to increased shielding effect by additional ten 4d electrons, which outweighs the effect of increased nuclear charge.
On moving from In to Tl, ionisation enthalpies show the increase again because fourteen 4f electrons shield valence electron poorely (order of shielding effect s>p>d>f) and so effective nuclear charge increases, consequently ionisation enthalpies
The ionization enthalpy values are as follows:
B= 801 kJmol-1, Al=577 kJmol-1 , Ga=579 kJmol-1, In=558 kJmol-1, and Tl=589 kJmol-1
Hint: Gallium has less ionization energy than aluminium