Ionic Solids, Which Have Anionic Vacancies Due to Metal Excess D Intext Questions Chapter 1 the Solid State Chemistry

Question 1.22: Ionic solids, which have anionic vacancies due to metal excess defect, develop colour.Explain with the help of a suitable example.

Answer
If ionic solid has metal excess defect due to anionic vacancies then ionic solid generate colur due to F –centers. Electrons absorb energy from the visible part of radiation and get excited.
For example, If the crystals of NaCl are heated in an atmosphere of sodium vapours, the sodium atoms deposited on the surface of the crystal .Chlorine ions comes outside the surface of crystal and react with this deposited sodium .Na atoms on the surface lose electrons to form Na+ ions and released electron occupy the vacant anionic sites. These electrons get excited by absorbing energy from the visible light falling on the crystal and generate yellow colour.