The colour of the `X_(2)` molecules of group 17 elements changes gradually from yellow to violet down the group. This is due to

Options

A. Decreases in `overset(**)(pi)-overset(**)(sigma)` gap down the group
B. Decreases in ionization energy down the group
C. The physical state of `X_(2)` at room temperature changes from gas to solid down the group
D. Decreases in HOMO-LUMO gap down the group

Answer

A, D

Explanation

`X_(2): sigma 1s^(2),sigma^(**)1s^(2),sigma2s^(2),sigma^(**)2s^(2),sigma2p_(z)^(2),pi 2p_(x)^(2) =pi 2p_(y)^(2) (overset(**)(pi)2p_(x)^(2)=overset(**)(pi)2p_(y)^(2))/("(HOMO)"), (sigma^(**)2p_(z)^(0))/("(LUMO)")`
`(F_(2))` Down the group `pi^(**) ` to `sigma^(**)` excitation become easy.