Question

F-centres in an ionic crystal is:
A. Lattice sites containing electrons.
B. Interstitial sites containing electrons.
C. Lattice sites that are vacant.
D. Interstitial site containing electrons.

Answer 1

F-centres arises due to anion vacancies. They are responsible for imparting colours to the crystal. Also, F in the name represents “Farbe” which translates to colour in German.

Complete step by step answer: For any crystal, if there is any departure from a perfectly ordered arrangement of constituent particles then we can say that defect or imperfection exists.
The defects are of basically two types:
- Point defect
- Line defect
- The point defect is further classified as:
- Stoichiometric defect
- Non-stoichiometric defect
- Impurity defect
F-centres arise due to metal excess defect which is classified under non-stoichiometric defect. If a negative ion is missing from its lattice site thus leaving a hole which is occupied by an electron thereby, maintaining the electrical balance then this site containing electrons thus trapped in the anion vacancy is called F-centres. In this, the hole that arises due to a negative ion missing from its lattice site is occupied by an electron to maintain the electrical neutrality.

Hence we can say F- centres in an ionic crystal are interstitial sites containing electrons.
So the correct answer is option D.
For example:
When $NaCl$ the crystal is heated in an atmosphere of sodium vapour, the excess of sodium atoms deposits on the surface of the crystal. $C{l^ - }$ ions then diffuse to the surface of the crystal where they combine with a sodium atom which becomes ionized by losing electrons. These electrons diffuse back to the crystal and occupy the vacant site created by $C{l^ - }$ ions.

Note: This effect is similar to the Schottky defect and it is found in a crystal having Schottky defect i.e. in ionic crystal having high coordination number and a small difference in the size of cations and anions.