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

Explanation

In ionic solid (e.g. `Na^+CI^-)` crystal, upon heating in the atmosphere of sodium (Na) vapour sodium atoms get deposited on the surface of the crystals. The `CI^-` ions from the crystal. Iaiti leave their sites and diffure into surface. They combine with sodium atoms present in vapou which in tum get ionized to from `Na^+` ·ions by releasing electrons. These electrons occupy the anionic sites are called F- centres (F =.Farbe meaning colour) Now, the electrons absorb radiatim of suitable wavelengths from visible white light and therefore appeared coloured