InterviewSolution
Saved Bookmarks
InterviewSolution
| 1. |
Write short note on metal excess and metal deficiency defect with an example. |
|
Answer» Solution :Metal excess defect Metal excess defect arises due to the presence of more number of metal ions as compared to ANIONS. Alkali metal halides NaCl, KCI show this type of defect. 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 extra cation and electron present in interstitial position. 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. Chloride ions `(CI^(-))` diffuse to the surface from the lattice point and combines with `Na^(+)` ion. The electron lost by the sodium vapour diffuse into the crystal lattice and occupies the vacancy created by the `CL^(-)`ions. Such anionic vacancies which are occupied by unpaired electrons are called F centers. Hence, the formula of NaCl which contains excess `Na^(+)` ions can be written as `Na_(1+x) Cl`. Metal deficiency defect :  Metal deficieney defect arises due to the presence of less number of cations than the anions. This defect is observed in a crystal in which, the cations have variable oxidation states. For example, in FeO crystal, some of the `FE^(2+)` ions are missing from the crystal lattice. To maintain the electrical neutrality, twice the number of other `Fe^(2+)` ions in the crystal is oxidized to `Fe^(3+)` ions, In such cases, overall number of `Fe^(2+)` and `Fe^(3+)` ions is less than the `O^(2-)` ions. It was experimentally found that the general formula of ferrous oxide is `Fe _(x)O`, where x ranges from 0.93 to 0.98. 
|
|