CBSE Class 12-science Answered

when the crystals are heated in Na vapour, the Na atoms are deposited on the surface of the crystal. The Cl^- ions diffuse to the surfaceand combine with Na to give NaCl. This happens by the loss of e^- by Na atoms to form Na⁺ ions. The released e^- difuse into the crystaland occupy anionic sites. As a result the crystal has excess of Na. the anionic sites occupied by the unpaired e^- are called F-Centers . They impart yellow colour to NaCl. Pink to LiCl and lilac to KCl.