A negative ion may be missing from its lattice site, leaving a hole which is occupied by an electron, thereby maintaining the electrical balance. The electrons thus trapped in the anion vacancies are called F-centres because they are responsible for impartingcolour to the crystals.For example, when NaCl is heated in an atmosphere of Na vapour, the excess of Na atoms deposit on the surface of NaCl crystal.
The Cl‌– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electrons 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 are occupied by unpaired electrons.
They impart yellow colour to the crystals of NaCl. The colour results by the excitation of these electrons when they absorb energy from the visible light falling on crystals.