Ionic solids which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.

Explanation

Metal excess defect is due to anionic vacancies. When crystals of an ionic solid like NaCl are heated in an atmosphere of sodium vapours, sodium atoms are deposited on the surface of the crystal. The `Cl^(-)` ions diffuse to the surface of the crystal and combine with `Na^(+)` ions to form NaCl. The `Na^(+)` ions are formed by the loss of electrons. These release electrons diffuse into the crystals and occupies anionic sites. The anionic sites occupied by the electrons are known as F-centre. They impart yellow colour to the crystal.