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

Explain this deviation from the general trend.

Explanation

Down the group `(darr)`, from `B` to `Al`, the ionisation enthalpy decreases due to increase in size and screening effect. Ionisation enthlypy of `Ga` is slightly higher than `Al, Ga` and In. These deviations from the general trend can be explained on the basis of electronic configuration.

`B` and `Al` have nobal gas core beneath the valence shell electrons, but at `Ga`, there is inclusion of lesser shielding fully filled `3d` orbital in between the noble gas core and the valence shell electrons, which results in increase in effective nuclear charge and valence shell electrons in effective nuclear charge and valence shell electrons are tightly held by the nucleus and hence higher energy is required for their removal. Hence, there is an increase in ionisation enthalpy frm `Al` to `Ga`. At `Tl`, fully filled lesser shielding `4f` as well as `5d` orbitals are present which result in much higher effective nuclear charge at `Tl`, Hence, `IE_(1)` is much higher than `Al, Ga` and In.