Question

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

Answer 1

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.