Question

The correct statement(s) regarding defects in solid is(are)

• A. Schottky defect occurs when large difference in the sizes of cations and anions.
• B. Frenkel defect is also know as dislocation defect
  
• C. F-centres are formed by anion vacancy of metal excess defect.
• D. Schottky defect have no effect on the physical properties of solids

Answer 1

Answer: (B), (C)

F-centres are formed by anion vacancy of metal excess defect.

The correct answers are (b) and (c).

When an ion is displaced from its regular position to an interstitial position, it creates a vacancy. This defect is called Frenkel defect. This is a kind of dislocation of ions thus, it is also known as dislocation defect.
Thus, statement (b) is correct.

Schottky defect arises when a pair of one cation and one anion of equal valency is missing from an ionic crystal, thus a pair of vacant sites are formed. These ion–pair vacancies decrease density of the solid. Hence, the physical properties of the solids change.
Thus, statement (d) is incorrect.

This defect is observed in ionic crystals having small difference in the sizes of cations and anions.
Thus, statement (a) is incorrect.

In metal excess defect, a negative ion may be absent from its lattice site, leaving a “hole” which is occupied by an electron.
These anionic sites occupied by unpaired electrons are called
F-centres.
Thus, statement (c) is correct.