Write short note on metal excess and metal deficiency defect with an example .

Explanation

Metal excess defect `:`
(i) Metal excess defect arises due to the presence of more number of metal ions as compared to anions.
Ex`:` NaCl, KCl
(ii) The electrical neutrality of the crystal can be maintained by the presence of anionic vacancies equal to the excess metal ions ( or) by the presence of of extra cation and electron present in interstitial position.
(iii) For example, when NaCl crystals are heated in the presence of sodium vapour, `Na^(+)` ions are formed and are deposited on the surface of the crystal.
(iv) Chloride ions `( Cl^(-))` diffuse to the surface from the lattice point and combines with `Na^(+)` ion.
(v) The electron lost by the sodium vapour diffuse into the crystal lattice occupies the vacancy created by the `Cl^(-)` ions.
(vi) Such anionic vacancies which are occupied by unpaired electrons are called F centers.
(vii) Hence, the formula of NaCl which contains excess `Na^(+)` ions cna be written as `Na_(1+x)Cl`.

Metal deficienty defect `:`
(i) Metal deficiency defect arises due to the presence of less number of cations than the anions.
(ii) This defect is observed in a crystal in which, the cations have variable oxidation states.
(iii) For example, in FeO crystal, some of the `Fe^(2+)` ions are missing from the crystal lattice.
(iv) To maintain the electrical neutrally, twice the number of other `Fe^(2+)` ions in the crystal is oxidized to `Fe^(3+)` ions.
(v) In such cases, overall number of `Fe^(2+)` and `Fe^(3+)` ions is less than the `O^(2-)` ions.
(vi) It was experimentally found that the general formula of ferrous oxide is `Fe_(x) O`,where x ranges from 0.93 to 0.98.