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

Explain this deviation from the general trend.

Explanation

On moving down the group `13` from `B` to `Al`,`IE` decreases as expected due to an increase in atomic size and screening effect which outweigh the effect of increased nuclear charge. However, `IE_(1)` of `Ga` is only slighly higher `(2 kJ mol^(-1))` than that of `Al` while that of `TI` is much higher than those of `Al`,`Ga` and `In`. These deviations are due to :
`Al` follows immediately after `s`-block elements and `TI` after d-and f-block elements. These extra `d-` and `f-` electrons do not shield (or screen) the outer shell electrons from the nucleus very effectively. As a result, the valence electrons remain more tightly held by the nucleus and hence larger amount of energy is needed for their removal. This explains why `Ga` has higher `IE` than `Al`. 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 `IE_(1)` of In is 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 `IE_(1)` of `TI` is higher than that of `In`.