Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitale example.
When crystals of NaCl are heated in an atmosphre of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The CI– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss electron by sodium atom to form Na+ ions. The released electrons diffuse into the crystals and occupy anionic sites. As a result the crystal now has an excess of sodium. They impart yellow colour to the crystals of NaCl.
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