Assertion: In Frenkel defect, dislocation of cation takes place from original site to interstitial site.

Reason: Density of the crystal is lowered.

Assertion is correct statement, but reason is wrong statement.

The correct answer is option 3. Assertion is correct statement, but reason is wrong statement.

Let us explain Frenkel defect and its relationship with the assertion and reason provided:

Frenkel defect is a type of point defect observed in ionic crystals where a cation vacancy and a cation interstitial are simultaneously formed. This occurs when a cation (typically a smaller ion) is dislocated from its original lattice site to an interstitial site within the crystal lattice. As a result, a vacancy is created at the original lattice site, and the displaced cation occupies an interstitial site.

Explanation of the Assertion:
The assertion states that in Frenkel defect, dislocation of the cation occurs from the original lattice site to the interstitial site. This is indeed correct. In Frenkel defect, the cation is displaced from its original position due to thermal vibrations or other factors and occupies an interstitial site, thereby creating a cation vacancy at its original lattice site.

Explanation of the Reason:
The reason provided states that the density of the crystal is lowered due to Frenkel defect. However, this is not accurate. Frenkel defect does not lead to a change in the density of the crystal. Even though a cation is displaced from its original lattice site, it remains within the crystal lattice, occupying an interstitial site. Therefore, the overall density of the crystal remains unchanged.

While the assertion correctly describes the phenomenon of Frenkel defect, the reason provided is incorrect. Frenkel defect does not result in a decrease in the density of the crystal. Instead, it maintains the overall density of the crystal while creating cation vacancies and cation interstitials. So, in summary, the assertion is correct, but the reason is incorrect.