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Question
Ionisation enthalpy (∆iH1kJ mol–1) for the elements of Group 13 follows the order.
Options
B > Al > Ga > In > Tl
B < Al < Ga < In < Tl
B < Al > Ga < In > Tl
B > Al < Ga > In < Tl
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Solution
B > Al < Ga > In < Tl
Explanation:
On moving down the group from B to Tl, a regular decreasing trend in the ionisation energy values is not observed.
| Element | B | Al | Ga | In | Tl |
| I.E. (kJ/mol) | 801 | 577 | 579 | 558 | 589 |
In Ga, there are ten d-electrons in the penultimate shell which screen the nuclear charge less effectively and thus, outer electron is held firmly. As a result, the ionisation energy of both A1 and Ga is nearly the same. The increase in ionisation energy from In to Tl is due to poor screening effect of 14f electrons present in the inner shell.
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