Explain using an appropriate example of how ionic solids, with anionic vacancies caused due to the metal excess defect, start developing a colour
When NaCl crystals are heated in the presence of sodium vapour some chloride ions left the lattice sites and combine with sodium to form NaCl. For this combination sodium atoms lose electrons to form Na+ ions. The released electrons diffuse into crystal to occupy the anion vacancies which are created by Cl− ions. In this condition the crystal have excess of sodium. The sites occupied by unpaired electrons are called F-centres or colour centres. F-centre imparts yellow colour to NaCl crystal because they absorb energy from the visible light and get excited.
