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When a crystal is exposed to white light, electrons trapped in vacancies absorb photons to transition from their ground state to an excited state, resulting in the compound appearing colored.

1. a) If a negative ion is missing from the lattice of an alkali metal halide, an electron may occupy the vacancy to preserve charge balance.
2. b) The vacancies filled by electrons are referred to as F-centres or color centers.
3. c) Electrons in these vacancies absorb light photons, transitioning to higher energy levels.
4. d) This absorption of light is responsible for the coloration of the crystal.

Correct answer: b) The vacancies filled by electrons are referred to as F-centres or color centers.

Solution

The correct option is B because F-centres are the 'holes' occupied by electrons, which are responsible for the color in crystals due to photon absorption.

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