The first ionization enthalpy values (in `Kj mol^(-1))` of group 13 elements are :
`{:(B,,A1,,Ga,,In,,Ti),(801,,577,,579,,558,,589):}`
How will you explain deviation from the general trend ?

Explanation

On moving down the group 13 from B to A1, ionization enthalpy decreases as expected due to an increases in atomic size and screening effect (of s- and e-electron of the 2nd shell ) which outweigh the effect of increased nuclear charge. However, `Delta_(i)H_(1)` of Ga is only slightly higher (2 kJ `mol^(-1)`) then that of A1 while that of T1 is much higher than those of A1, Ga and In. These deviations can be explained and follows:
A1 follows immediately after s- element while Ga and In follows after d- block elements and T1 after d- and f- block elements . Since d- and f- electrons shield the valence electrons from the nucleus less effectively than s- and p- electrons therefore valence electrons remain fairly strongly attracted by the nucleus . As a result effective nuclear charge increases and consequently ionization ethalpy increases
Thus as we move from A1 to Ga due to ineffective shielding of the valence electrons by 3d- electrons the effective nuclear charge on Ga is slightly more than on A1. As a result `Delta_(i) H_(1)` of Ga is slightly higher than that of A1. Further on moving down the group from Ga to In, the nuclear charge increases by 18 units (49 -31 =18) . Here the shielding effect of all the inner electrons outweighs the effect of the the nuclear charge and hence `Delta_(I)H_(1)` of In is lower than that of Ga. On Further moving from In to T1 the nuclear charge further increases by 32 units (81- 49 = 32) this increased nuclear charge outweighs the shielding effect of all the electrons present in the inner shells including those of 4f and 5d- electrons and hence `Delta_(i) H_(1) ` of T1 is higher than that of In.