Ionisation enthalpy `(Delta_(i)"H kJ mol"^(-1))` for the elements of group 13 follows the order.

Options

A. B `gt` Al `gt` Ga `gt` In `gt` Tl
B. B `lt` Al `lt` Ga `lt` In `lt` Tl
C. B `lt` Al `gt` Ga `lt` In `gt` Tl
D. B `gt` Al `lt` Ga `gt` In `lt` Tl

Answer

D

Explanation

On moving from B to Al, all the ionisation ethalpies decreases as expected and this decrease is due to an increase in atomic size and shielding effect.
On moving from Al to Ga, the ionisation enthalpy increases sightly, because on moving from Al to Ga, both nuclear charge and shielding effect increase but due to poor shielding by d-electron in Ga, effective nuclear charge on valence electron increases resulting in d-block contraction, that's why ionisation enthalpies increase.
On moving from Ga to ln, again there is slight decrease in ionisation enthalpies due to increased shielding effect by additional ten 4d electrons, which outweighs the effect of increased nuclear charge
On moving from ln to Tl, ionisation enthalpies show the increase again because fourteen 4f electrons shield valence poorely (order of shieling effect s `gt` p `gt` d `gt` f) and so effective nuclear charge increases, consequently ionisation ethalpies increase.