Why does KCl turn violet on heating in the potassium vapor?

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Yes u ar correct.

It is because of the Metal excess defect due to anionic vacancies.

When crystals of KCl are heated in an atmosphere of potassium vapour, the potassium atoms are deposited on the surface of the crystal. The Cl– ions diffuse to the surface of the crystal and combine with K atoms to give KCl. This happens by loss of electron by sodium atoms to form K+ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result the crystal now has an excess of potassium. The anionic sites occupied by unpaired electrons are called F-centres (for colour centre). They impart violet colour to the crystals of KCl. The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.

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  1. It is because of the Metal excess defect due to anionic vacancies.
  2. When KCl is heated with K(vap) the Cl- ions diffuse from the crysta to the surface in the K atoms form the K 9vap) are deposited.
  3. Both these combine together to form KCL.
  4. Further in the crystal there is an excess of Katoms along with the unpaired electrons.
    KCl --------> K+ + Cl-
  5. The unpaired electrons occupy the anionic sites, these sites are also called as F-Centres.
  6. When visible light passes through this F-Centres the electrons gets excited i.e., absorb some energy and inheret some charaterstic colour, in case of KCl it is violet.
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Metal excess defect due to anion vacancy Sorry friend the above picture is wrong........this one is the correct one.......Sorry fr the incomvenience.

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