Why does sodium chloride on heating with sodium vapours acquire yellow colour?
NaCl shows metal excess defect due to anionic vacancies. 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 the loss of electrons by sodium atoms to form Na+ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result, crystal now has excess Na+ ions. The anionic sites occupied by unpaired electrons are called F–centres. They impart the yellow colour to the crystals of NaCl. The colour results by excitation of these electrons when they absorb energy from visible light falling on the crystal.