1.

The colour of the X2 molecules of group 17 elements changes gradually from yellow to violet down the group. This is due to (A) the physical state of X2 at atom temperature changes from gas to solid down the group (B) decrease in HOMO-LUMO gap down the group (C) decrease in π*-σ* gap down the group (D) decrease in ionization energy down the group

Answer»

(B) decrease in HOMO-LUMO gap down the group 

(C) decrease in π*-σ* gap down the group 

Highest occupied molecular orbital (HOMO) ⇒ π* 

Lowest unoccupied molecular orbital (LUMO) ⇒ σ* 

On descending the group gap between π* and σ* decreases.



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