False statement about Frenkel defect is:

Non-ionic solids show this defect

The correct answer is option 3.Non-ionic solids show this defect.

The Frenkel defect is a type of point defect observed in ionic crystals. It involves the movement of an ion within the lattice, typically occurring due to the following reasons:

Vacancy Creation: An ion (usually a cation) leaves its normal lattice site, creating a vacancy at that position.

Interstitial Formation: The ion that has left its original position moves to an interstitial site (a space between the normal lattice positions), creating an interstitial defect.

This defect is observed in crystals where there is a significant size difference between the cations and anions.

Analysis of Each Statement

1. It is a combination of vacancy defect and interstitial defect.

The statement is true. The Frenkel defect indeed involves both a vacancy and an interstitial defect:

The cation leaves its lattice position, creating a vacancy.

The cation moves to an interstitial site within the crystal structure.

Thus, this statement correctly describes the nature of the Frenkel defect.

2. Density remains unchanged.

The statement is true. The total number of ions in the crystal remains the same; ions are merely relocated from lattice sites to interstitial sites. Since density is the mass per unit volume, and both mass and volume remain effectively unchanged by the Frenkel defect, the density of the crystal remains constant. Thus, this statement is also true.

3. Non-ionic solids show this defect.

The statement is false. The Frenkel defect is specific to ionic solids, particularly those with a large size difference between cations and anions. This type of defect is not commonly found in non-ionic solids. In non-ionic solids, other types of defects (e.g., vacancies, dislocations) are more prevalent, and the Frenkel defect is not a typical characteristic. Therefore, this statement is incorrect.

4. Shown by solids with a large difference in size of cations and anions.

The statement is true. The Frenkel defect is particularly common in ionic solids where the cations and anions have a significant size difference. This size disparity allows the smaller cation to move into interstitial sites more easily, which is the basis for the Frenkel defect.

Thus, this statement correctly describes the conditions under which a Frenkel defect is observed.

Summary

The Frenkel defect is a combination of a vacancy and an interstitial defect and occurs in ionic solids where there is a considerable size difference between cations and anions. The density of the solid remains unchanged, and the defect is not observed in non-ionic solids. Therefore, the false statement is: Option 3: Non-ionic solids show this defect.