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

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In metal excess defect F-centres are generated. F-centres are the anionic sites occupied by unpaired electrons. These F-centres are responsible for the developing of colour.

For e.g. when crystals of KCl are heated in an atmosphere of sodium vapour, the potassium atoms comes on the surface of the KCl. Then the Cl– ions go to the surface of the KCl and get combined with K atoms to give KCl. This happens by loss of an electron by potassium atoms to form K+ ions. The released electrons during the process go into the KCl and occupy anionic sites. Thus the crystal now has an excess of potassium. Generation of F-centres takes place. They impart violet colour to the crystals of KCl. 

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

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