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Going down group 17, the colour of the diatomic X2 molecules deepens from yellow to violet. The reason for this trend is

1. the physical state of X2 at room temperature changes from gas to solid down the group
2. decrease in HOMO-LUMO gap down the group
3. decrease in pi*-sigma* gap down the group
4. decrease in ionization energy down the group

Correct answer: decrease in HOMO-LUMO gap down the group

Solution

Down the group the HOMO-LUMO (pi* to sigma*) energy gap decreases, so the molecules absorb progressively longer-wavelength visible light, shifting the colour from yellow toward violet.

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