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

Explanation

(i) The colour develoops because of the present of electrons in the 8 anionic sites.
(ii) These electrons absorb energy from the visible region of radiation and get excited.
(iii) For example when crystals of NaCl are heated in an atmosphere of sodium vapours, the sodium atoms get deposited on the surface of the crystal and the deposited Na atoms.
(iv) During this process, the Na atoms on the surface lose electrons to form `Na^(+)` ions and the released electrons diffuse into the crystal to occupy the vacant anionic sites.
(v) These electrons get excited by absorbing energy from the visible light and impart yellow colour to the crystals.