JEE Advanced · 2017 · 21 May 2017 · Paper 1 · p-Block Elements

The colour of the

X_{2}

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

(A)the physical state of $X_{2}$ at room temperature changes from gas to solid down the group
(B)decrease in ionization energy down the group
(C)decrease in $π^{*} - σ^{*}$ gap down the group✓ Correct
(D)decrease in HOMO-LUMO gap down the group✓ Correct

Answer: (C), (D)

Solution

The colour of halogens deepens down the group (

F_{2}

pale yellow →

I_{2}

violet) because the energy gap between the highest occupied molecular orbital (HOMO, which is

π^{*}

) and the lowest unoccupied molecular orbital (LUMO, which is

σ^{*}

) decreases.

As the HOMO-LUMO gap (or

π^{*} - σ^{*}

gap) decreases, the wavelength of light absorbed increases, shifting the colour from yellow to violet.

Correct answers: C, D

JEE Advanced · 2017 · 21 May 2017 · Paper 1 · One or more correct options