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When a crystal of NaCl is heated in sodium vapour, it acquires a yellow colour. The yellow colour is due to non stoichiometric defect. Name the defect.

When a crystal of NaCl is heated in sodium vapour, it acquires a yellow colour. The yellow colour is due to non stoichiometric defect. Name the defect.

Grade:upto college level

3 Answers

Sunil Kumar FP
askIITians Faculty 183 Points
6 years ago
the yellow colour acquired by the NaCl crystal is due to defect called metal excess defect due to anionic vacancy.
When crystals of NaCl is heated in an atmosphere of sodium vapour the sodium ion are deposited on the surface of crystal.The Cl- ion diffuse to form NaCl.This happens by loss of electron by Na to form Na+.The released electron diffuse into the crystal and occupy anionic site.these are called F centre and it imparts yellow colour to NaCl
Ayoun Ghosh
34 Points
4 years ago
Metal excess defects due to anion vacancy: this type of defect arises when an anion is missing from its lattice site, creating a hole, and the hole is occupied by an electron to maintain the electrical neutrality of the crystal.
T shanmuka rao
15 Points
one year ago
Any alkali halide crystals will show coloured center when this type of crystal are heated with the metal or halogen vapour 
But in the case of NaCl crystal the metal is Na so this Nacl crystal is heated with this Na metal vapour there is an adsorption of Na atom at the surface . And the electron is separated from  the Na atom and become Na+ ion to make a surface or to maintain neutrality the inner Cl- ion comes to the surface and the electron from the Na atom is diffuses into the crystal and when quenched to the room temprature then form a Fcenter. 
So F center is the electron trapped in between  six cation then this Nacl crystal shows brownish yellow
Licl shows pink
kcl shows voilet etc. 
This things are obtained in non stoichiometric crystal i,e containing excess of one of its constituent ion.. 
This absorption band shows colour in uv and visible region. 
 

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