Question

The colour of $X_2$ 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. The decrease in HOMO-LUMO gap down the group
• C. The decrease in ${\pi }^{\ast }-{\sigma }^{\ast }$ gap down the group
• D. The decrease in ionization energy down the group

Answer 1

Answer: (B), (C)

The correct options are
B The decrease in HOMO-LUMO gap down the group
C The decrease in ${\pi }^{\ast }-{\sigma }^{\ast }$ gap down the group




Highest occupied molecular orbital (HOMO) $\Rightarrow np{\pi }^{\ast }$

Lowest unoccupied molecular orbital (LUMO) $\Rightarrow np{\sigma }^{\ast }$

Down the group, the energy gap between $np{\pi }^{\ast }$ and $np{\sigma }^{\ast }$ decreases.

The colour of these molecules are due to the electronic excitation which occur by the of absorbtion of visible light and the wavelength of absorbed light depends on the energy gap between HOMO and LUMO.

Fluorine appears yellow by absorbing violet light and iodine appears violet by absorbing yellow light.

${\lambda }_{violet}<{\lambda }_{yellow}$ , fluorine as it absorbs violet light means that the HOMO-LUMO gap is maximum, and down the group, the energy gap decreases.