Question

What is the metal excess defect? Explain with examples.

Answer 1

Metal excess defect:

• It is a type of defect observed in crystals in which excess of metal occurs.
• There are two types of metal excess defects.

(a) Metal excess defect due to anionic vacancies:

• This defect occurs in alkyl halides such as $\mathrm{NaCl}$ and $\mathrm{KCl}$.
• In this kind of defect, the negative ion is found to be missing from its lattice site making a hole which gets occupied by an electron to maintain electrical neutrality.

For example:

• When Sodium chloride ($\mathrm{NaCl}$) crystals are heated in the vapors of Sodium ($\mathrm{Na}$), sodium ions get deposited on the surface on the crystal.
• Now, Chloride ions (${\mathrm{Cl}}^{-}$) move to the surface to get combined with Sodium ions (${\mathrm{Na}}^{+}$) to form $\mathrm{NaCl}$.The lattice from where the Chloride ions are displaced now becomes empty which is occupied by the electrons released when Sodium atoms ($\mathrm{Na}$) are converted into Sodium ions (${\mathrm{Na}}^{+}$).
• Thus, as a result, the crystal has an excess of Sodium metal.

(b) Metal excess defect due to the presence of extra cations at interstitial sites:

• In this defect, extra cations are released on heating the compound.
• These cations occupy the interstitial sites.

For example:

• When white Zinc oxide ($\mathrm{ZnO}$) is heated, it loses oxygen and becomes yellow.

$\mathrm{Zn}\stackrel{\mathrm{heating}}{\to }{\mathrm{Zn}}^{2+}+\frac{1}{2}{\mathrm{O}}_2+2{\mathrm{e}}^{-}$

• Now, an excess of Zinc occurs in the crystal and its formula becomes ${\mathrm{Zn}}_{1+\mathrm{x}}\mathrm{O}$.