Question

Non stoichiometric nacl is yellow in colour. why?

Answer 1

They impart yellow colour to the crystals of NaCl. Thecolour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.

Answer 2

Metal excess defect due to anionic vacancies can explain this occurrence.

It's a non-stoichiometric flaw that's most common in alkali halides. When table salt crystals are heated, a sodium vapour environment formed, and Na+ atoms are deposited on the crystal's surface. Cl– ions diffuse to the crystal's surface, where they interact with Na+ atoms to form NaCl. The freed electrons disperse within the crystal and take up residence in anionic sites. As a result, there is now an overabundance of sodium in the crystal. F-centres are anionic locations populated by unpaired electrons (German word Farbenzenter for colour centre).

Because these electrons are excited when they absorb energy from visible light falling on the crystals, they give the NaCl crystals a yellow colour. As a result, at high temperatures, NaCl can turn yellow.

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