Question

An excess of potassium ions makes KCL crystals appear violet or lilac in colour since …………. .
A. some of the anionic sites are occupied by an unpaired electron.
B. some of the anionic sites are occupied by a pari of electrons
C. there are vacancies at some anionic sites
D. F-centres are created which impart colour to the crystals.

Answer 1

Correct Answer - A,D
When KCl is heated in vapour of K, some of the `Cl^(-)` leave their lattice site and create anion vacancies. This chloride ion wants to combine with K vapour to form potassium chloride. For doing so K. atom loses electrons form `K^(+)` ions. This released electron diffuses into the crystal to get entrapped in the anion vancancy called F-centre. when visible light falls on the crystal, this entrapped electron gains energy, goes to the higher level when it comes back to the ground state, energy is released in the form of light.