pBlock ElementsII 2 Question 23
23. The colour of the $X_{2}$ molecules of group 17 elements changes gradually from yellow to violet down the group. This is due to
(2017 Adv.)
(a) decrease in $\pi^{*}-\sigma *$ gap down the group
(b) decrease in ionisation energy down the group
(c) the physical state of $X_{2}$ at room temperature changes from gas to solid down the group
(d) decreases in HOMO-LUMO gap down the group
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Solution:
- Colour of halogen arises due to transition from HOMO to LUMO
| Compounds | Hydridisation | Structures | Lone pair on central atom |
|---|---|---|---|
| $\mathrm{BrF}_{5}$ | $s p^{3} d^{2}$ | 1 | |
| $\mathrm{ClF}_{3}$ | $s p^{3} d$ | 2 | |
| $\mathrm{XeF}_{4}$ | $s p^{3} d^{2}$ | 2 | |
| $\mathrm{SF}_{4}$ | $s p^{3} d$ | 1 |
in the visible region. On moving down a group, the difference in energy between HOMO and LUMO decreases electronic transition occur more easily and colour intensity increases.
