When crystals of sodium chloride are heated in the presence of sodium vapours they turn yellow,this is due to

When NaCl is taken in the atmosphere of Na vapours,some of the Na atoms are deposited on the surface of NaCl.These deposited Na vapour atoms on reaction with Cl- ions gives Na+Cl-+e- .This e-  will occupy the vacancy which is created by Cl- .That is the electron occupies the anion vacancies.This is said to be F-centreDue to this F-centre NaCl gets yellow colour  {farbe=colour}.                                             Na+Cl-→Na+Cl-+e-Na+  Cl-  Na+   Cl- Cl-   Na+  Cl-   Na+ Na+  e-    Na+  Cl-