Grade upto college level Physical Chemistry

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.

aditya kashyap

12 Years agoGrade upto college level

3 Answers

Sunil Kumar FP

11 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

9 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

6 Years 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.