The colour of the X 2 molecules of group 17 elements changes gradually from yellow to violet down the group. This is due to -

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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 ${Π^{*}}_{2 p_{x}}$ to ${σ^{*}}_{2 p_{z}}$ therefore colour intensify.

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