The appearance of colour in solid alkali metal halides is generally due to

Options

A. Interstital positions
B. F- centres
C. Schottky defect
D. Frenkel defect

Answer

2

Explanation

Alkali halides like `NaCl` and `KCl` show metal excess defect due to anionic vacancies ,When crystal of `NaCl` are heated in an atmosphere of sodium vapour the sodium atoms are depostied 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 Valance electron by `Na` atoms to form `Na^(+)` ions. The relased electrons diffuse into the crystal and occupy anionic sites.
As a result the crystal now has an excess of sodium . The anionic sites occupied by unpaired electrons are called `F` -centres ( form the German word, Farbenzenter for colour centre). They impact yellow colour to the crystals of `NaCl`. The colour results by excitaiton of these electorns when they absorb energy from the visible light falling ont eh crystals. Similarly excess of `Li` makes `LiCi` crystals pink, and excess of `K` makes `KCl` crystal violet (or lilac).