Q. 445.0( 1 Vote )

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 pair of electrons.

C. there are vacancies at some anionic sites.

D. F-centres are created which impart colour to the crystals.

Answer :

When excess of KCl ions are heated, the colour of the substance appears violet or lilac. This phenomenon can be explained by metal excess defect due to anionic vacancies. When crystals of KCl are heated in an atmosphere of potassium vapour, the K+ atoms are deposited on the surface of the crystal. The Cl ions diffuse to the surface of the crystal and combine with K+ ions to give KCl. This happens by loss of electron by potassium atoms to form K+ ions. The released electrons diffuse inside the crystal and occupy anionic sites. The crystal ends up with an excess of potassium. The anionic sites occupied by the unpaired electrons are called F-centres. They impart lilac or violet colour to the crystals of KCl. The colour results due to excitation of these electrons when they absorb energy from the visible light falling on the crystals. The correct options are (i) and (iv).

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