Question
What makes alkali metal halides sometimes coloured, which are otherwise colourless?
Solution
Metal excess defect is resposible for colour in alkylhalides.
When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal.
The Cl– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na+ ions. The released electrons diffuse into the crystal and occupy anionic sites and impart colour.
