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

When $\mathrm{NaCl}$is taken in the atmosphere of $\mathrm{Na}$ vapours,some of the $\mathrm{Na}$ atoms are deposited on the surface of $\mathrm{NaCl}$.

These deposited $\mathrm{Na}$vapour atoms on reaction with ${\mathrm{Cl}}^{-}$ ions gives ${\mathrm{Na}}^{+}{\mathrm{Cl}}^{-}+{\mathrm{e}}^{-}$ .This ${\mathrm{e}}^{-}$  will occupy the vacancy 

which is created by ${\mathrm{Cl}}^{-}$ .That is the electron occupies the anion vacancies.This is said to be F-centre

Due to this F-centre NaCl gets yellow colour  {farbe=colour}.

                                             $\mathrm{Na}+{\mathrm{Cl}}^{-}\to {\mathrm{Na}}^{+}{\mathrm{Cl}}^{-}+{\mathrm{e}}^{-}$

$$\begin{gathered} {\mathrm{Na}}^{+} {\mathrm{Cl}}^{-} {\mathrm{Na}}^{+} {\mathrm{Cl}}^{-} \\ {\mathrm{Cl}}^{-} {\mathrm{Na}}^{+} {\mathrm{Cl}}^{-} {\mathrm{Na}}^{+} \\ {\mathrm{Na}}^{+} \boxed{{\mathrm{e}}^{-}} {\mathrm{Na}}^{+} {\mathrm{Cl}}^{-} \end{gathered}$$