The first ionization enthalpy values (in kJ mol^-1) of group 13 elements are:
B       Al      Ga      In        Tl
801    577   579    558    589
How would you explain this deviation from the general trend?

The decrease in first ionization enthalpy (∆_IH₁) value from B to Al is due to the bigger size of Al atom. But the element Ga has ten 3d electrons present in the 3d sub-shell which does not screen as much as is done by s and p electrons. Therefore there is an unexpected increase in the magnitude of effective nuclear charge resulting in increased ∆_iH₁, value. The same explanation can be given in moving from In to TT. The latter has fourteen 4/ electrons with very poor shielding effect. This also results in an unexpected increase in the effective nuclear charge resulting in inflated ∆_IH₁ value.