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Ionisation enthalpy (Delta(i)"H kJ mol"^...

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

A

`B gt Al gt Ga gt In gt TI`

B

`B lt Al lt Ga lt In lt TI`

C

`B lt Al gt Ga lt In gt TI`

D

`B gt Al lt Ga gt In lt TI`

Text Solution

Verified by Experts

The correct Answer is:
D
On moving from B to Al, all the ionisation enthalpies decreases as expected and this decreases is due to an increase in atomic size and shielding effect. On moving from Al to Ga, the ionisation enthalpy increases slightly, 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 In, again there is slight decrease in ionisation enthalpies due to increase shielding effect by additional ten 4 d electrons, which outweighs the effect of increased nuclear charge On moving from In to Tl, ionisation enthalpies show the increase again because fourteen 4 f electrons shield valence electron poorly (order of shielding effect `s gt p gt d gt f`) and so effective nuclear charge increases, consequently ionisation enthalpies increase.
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