Question:

The colour of the X₂ molecules of group 17 elements changes gradually from yellow to violet down the group. This is due to:

decrease in π*−σ gap down the group

the physical state of X₂ at room temperature changes from gas to solid down the group

decrease in ionization energy down the group

decrease in HOMO−LUMO gap down the group

Solution:

The colour of the X₂ molecules of group 17 elements changes gradually from yellow to violet down the group. This is due to (B) Decrease in π*−σ gap down the group and (C) decrease in HOMO−LUMO gap down the group. Halogens absorb visible light and appear coloured. Outer electrons are excited to higher energy level. Smaller halogen has higher excitation energy and vice versa.