When crystals of NaCl are heated with sodium vapour, the sodium ions are deposited on the surface of NaCl crystals and the Cl−ions diffuse to form NaCl. During the process the loss of electron from Na to form Na+ion takes place. The electron given out by Na diffuses into the crystal and occupies anionic site which is called F-centre. Therefore, the formation of F-centres is metal excess defect.
