Ionic solids, which have anioninc vacancies due to metal excess defect, developed colour. Explain with the help of a suitalbe example.

Explanation

When a crystals of alkali metal halides such as NaCl are heated in the atmosphere of sodium vapour, the excess of sodium atoms are deposited on the surface of the crystal. The `Cl^(-)` ions diffues to the surface and combine with sodium atoms to form `Na^(+)` ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result, the structure has an excess of sodium.
The crystals acquire yellow colour becuase when visible light falls on the crystals, they absorb energy from the visible region to excite the electrons. Hence, they are coloured. Similarly, excess of Li makes LiCl crystals pink excess of K makes KCl crystals violet.