Explain metal excess defects.

(N/A) $(i)$ Defects due to anionic vacancies: Alkali halides such as $NaCl$ and $KCl$ show this type of defect.
In this type of defect,an anion is missing from its lattice site,and the site is occupied by an electron to maintain electrical neutrality. These electrons trapped in anionic vacancies are called $F$-centers (from the German word $Farbenzentrum$,meaning color center),which are responsible for the color of the crystals.
$(ii)$ Defects due to the presence of extra cations at interstitial sites: Zinc oxide $(ZnO)$ is white at room temperature. On heating,it loses oxygen and turns yellow due to the following reaction:
$ZnO \xrightarrow{\Delta} Zn^{2+} + \frac{1}{2}O_2 + 2e^-$
Now,there is an excess of zinc in the crystal and its formula becomes $Zn_{1+x}O$. The excess $Zn^{2+}$ ions move to interstitial sites and the electrons to neighboring interstitial sites to maintain electrical neutrality.