First ionisation enthalpy values (in kJ `mol^(-1)`) of group-13 elements are B=801, Al=577, Ga=578, In=558 and TI=589, how would you explain this deviation from the general trend?

Explanation

On moving down group-13 from B to Al, ionisation enthalpy decreases due to increase in atomic size and shielding effect which jointly overcome the effect of increase in nuclear charge. However, ionisation entghalpy increases slightly on moving from Al to Ga (2 `kJ*mol^(-1)`). this is because due to poor shielding of valence electrons by 3d-electrons effective nuclear charge on Ga is slightly more than Al. on moving from Ga to 1n, the shileding effect of all the inner electrons overcomes the effect of the increase in nuclear charge. thus, ionisation enthalpy of In is lower than Ga. again, on moving from In to TI, there is further increase in nuclear charge which overcomes the shielding effect of all electrons present in the inner shells including those of 4f- and 5d-orbitals. so, ionisation enthalpy of Tl is higher than In.