The colour of the X 2 molecules of group 17 elements changes gradually from yellow to violet down the group.…

The colour of the X2 molecules of group 17 elements changes gradually from yellow to violet down the group. This is due to -
  1. Decrease in HOMO-LUMO gap down the group
  2. Decrease in π*-σ* down the group
  3. The physical state of  X2 at room temperature changes from gas to solid down the group
  4. Decrease in ionization energy down the group

Solution

Halogens are coloured due to HOMO-LUMO transition of electrons.

On moving down the group HOMO-LUMO energy gap decreases so transition of electrons become easier from  Π*2px to σ*2pz therefore colour intensify.

Asked in: JEE Advanced 2017 (Paper 1)

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