CBSE Class 6 Answered

When there is an excess of metal ions in non-stoichiometric compounds, the crystal lattice has vacant anion site. The anion sites occupied by electrons are called F-centre. The simplest and the most studied type of colour centre is an F centre. It is called an F centre because its name comes from the German word Farbe which means colour. F centres are generally produced by heating a crystal in an excess of an alkali vapour or by irradiating the crystal by X rays, NaCl is a very good example having F centres.

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.