Question

What makes alkali metal halides sometimes coloured, which are otherwise colourless?

Answer 1

1. Alkali metal halides, which are usually colourless in their pure form, sometimes appear coloured due to the presence of defects known as F-centres (or colour centres) in their crystal lattice.
2. An F-centre is formed when a halide ion vacancy (such as a missing Cl⁻ ion) is occupied by a free electron.
3. This electron, trapped in the anion vacancy, can absorb certain wavelengths of visible light.
4. The absorption of specific wavelengths and the transmission of the remaining light gives the crystal its characteristic colour.
5. These defects typically form when the halide is heated in the presence of excess alkali metal vapour, causing halide ions to leave their lattice sites and be replaced by electrons from the metal atoms.
6. For example, NaCl turns yellow, KCl becomes violet and LiCl appears pink due to these F-centers. 
7. Thus, the colour in alkali metal halides arises from electronic transitions associated with these crystal defects.