JEE Advanced 2017 · Paper 1 · Chemistry Q4

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ChemistryMultiple correct+4 / -2Question 22 of 54

The colour of the $\ce{X2}$ molecules of group 17 elements changes gradually from yellow to violet down the group. This is due to

the physical state of

\ce{X2}

at room temperature changes from gas to solid down the group

decrease in ionization energy down the group

decrease in

\pi^*

\sigma^*

gap down the group

decrease in HOMO-LUMO gap down the group

Answer and worked solution

Official answer: C, D

The colour of $\ce{X2}$ comes from a $\pi^*\to\sigma^*$ excitation, the HOMO $\to$ LUMO transition. Down the group this gap shrinks, so absorption moves to longer wavelength. The transmitted colour therefore shifts from yellow ( $\ce{F2}$ ) through green-yellow and red-brown to violet ( $\ce{I2}$ ).

Physical state and ionisation energy do not set the colour.

(C), (D)

Topics:Group 17 Molecular orbital theory

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JEE Advanced 2017 · Paper 1, Chemistry question 4