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When the crystal is irradiated with white light, the trapped electrons in a hole absorb photon for excitation from ground state to excited state. This gives colour to the compound.

1. a) Alkali metal halide may have excess metal ion if a negative ion is absent from its lattice site, leaving a hole, which is occupied by electron to maintain electrical neutrality.
2. b) The 'holes' occupied by electrons are called F-centres or colour centres.
3. c) When the crystal is irradiated with white light, the trapped electrons in a hole absorb photon for excitation from ground state to excited state.
4. d) This gives colour to the compound.

Correct answer: b) The 'holes' occupied by electrons are called F-centres or colour centres.

Solution

F-centres or colour centres are formed when a negative ion is absent from its lattice site, leaving a hole, which is occupied by an electron.

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