Solution

Alkali halides like $$NaCl$$ and $$KCl$$ show metal excess defect. When crystals of $$NaCl$$ are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The $$Cl^-$$ ions diffuse to the surface of the crystal and combine with $$Na$$ atoms to give $$NaCl$$. This happens by loss of electron by sodium atoms to form $$Na^+$$ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result, the crystal now has an excess of sodium. The anionic sites occupied by unpaired electrons are called F-centres. They impart yellow colour to the crystals of $$NaCl$$. 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 $$LiCl$$ crystals pink and excess of potassium makes $$KCl$$ crystals violet (or lilac).