The first ionisation enthalpy of group `13` elements are :

Explain this deviation from the general trend.

Answer

On moving down the group `13` from `B` to `AI`, the ionization enthalpy decreases as expected due to an increases in atomic size and screening effect, which outweighs the effect of increased nuclear charge. However `Delta_(i)H` of Gas is only slightly greater `(2 kJ mol^(-1))` then that of `AI`. while that of `TI` is much higher than in. These deviations can be explained as follows:
At follows immediately after s-block elements, while Ga-and in follow after d-block elements and `TI` after d-and f-block elements. These extra and f-electrons do not shiled (or screen) the outermost shell electrons from the nucleus very effectively As a result, the valence electron remian more tightly held be the nucleus and hence larger amount of energy is nedded for their removal. This explains whay `Ga` has higher ionization enthalpy than `AI`. Further, on moving down the group from `Ga` to in, the increased shielding effect (due to the presence of additional `4d`-electrons) outweighs the effect of increased nuclear charge `(49 - 31 = 18` units) and hence the `Delta_(i)H` of in lower than that of `Ga`. Thereafter, the effect of increased nuclear charge `(81 - 49 = 32`units) outweighs the shielding effect due to the presence of additional `4f` and `5d` electrons and hence the `Delta_(i)H` of `TI` is higher than that of in.