question_answer 22)

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

Answer:

Alkali halides like NaCI and KCI show this type of defect. When crystals of NaCI are heated in an atmosphere of sodium vapour, the sodium   atoms are deposited on the surface of the crystal. The  ions diffuse to the surface of the crystal and combine with Na atoms to give . This happens by loss of electron by sodium atoms to form ions. The released electrons diffuse into the crystal and occupy anionic sites (Fig.). As a result, the crystal now has an excess of sodium. Thus, it is metal excess defect. The anionic sites occupied by unpaired electrons are called F-centres (from the German word Farbenzenter for colour centre). They impart yellow colour to the crystals of . The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals. Similarly, excess of lithium makes  crystals pink and excess of potassium makes  crystals violet (or lilac).