Looking for classes? Ksquare Career Institute, Bengaluru →
The appearance of colour in solid alkali metal halides is generally due to :
A
F-centres
B
Schottky defect
C
Frenkel defect
D
Interstitial positions
Detailed Solution
When an alkali metal halide crystal is heated in the vapour of the alkali metal, metal atoms deposit on the surface.
Halide ions diffuse to the surface and combine with these atoms, and the electrons released by the ionising metal atoms diffuse into the crystal and occupy the vacant anion sites.
The anion vacancies occupied by unpaired electrons are called F-centres (from the German word Farbenzenter, meaning colour centre).
These electrons absorb energy from visible light and get excited, which gives colour to the crystal.
Because of the F-centre defect a colourless ionic crystal is converted into a coloured one: NaCl becomes yellow, KCl violet (lilac) and LiCl pink.
Schottky and Frenkel defects are stoichiometric defects and do not produce colour.
So the colour is generally due to F-centres.
Halide ions diffuse to the surface and combine with these atoms, and the electrons released by the ionising metal atoms diffuse into the crystal and occupy the vacant anion sites.
The anion vacancies occupied by unpaired electrons are called F-centres (from the German word Farbenzenter, meaning colour centre).
These electrons absorb energy from visible light and get excited, which gives colour to the crystal.
Because of the F-centre defect a colourless ionic crystal is converted into a coloured one: NaCl becomes yellow, KCl violet (lilac) and LiCl pink.
Schottky and Frenkel defects are stoichiometric defects and do not produce colour.
So the colour is generally due to F-centres.