Taking the example of NaCl, when its crystals are heated in presence of sodium vapour, some chloride jons leave their lattice sites to combine with sodium to form NaCl. For this reaction to occur, Na atoms lose electrons to form `Na^+` ions. The electrons thus released diffuse into the crystal to occupy the anion vacancies created by `Cl^-` ions. The crystal now has excess of sodium. The sites occupied by unpaired electrons are called F-centres. They impart yellow colour to the crystal because they absorb energy from the visible light and get excited
