Introduction:
When Frenkel defects are created in an otherwise perfect ionic crystal, the density of the ionic crystal remains the same. This is because Frenkel defects do not involve a change in the number of atoms in the crystal lattice, but rather a redistribution of atoms within the lattice.

Frenkel Defects:
Frenkel defects occur when an atom or ion in a crystal lattice moves from its regular position to an interstitial site, leaving behind a vacancy at its original position. This type of defect is commonly observed in ionic crystals where the cations are much larger than the anions.

Density of Ionic Crystal:
The density of a material is defined as its mass per unit volume. In the case of an ionic crystal, the mass is determined by the sum of the masses of the cations and anions present in the crystal lattice. The volume is determined by the arrangement of the ions in the lattice.

No Change in Mass:
When a Frenkel defect occurs, an atom or ion moves from its original position to an interstitial site. This movement does not change the number of atoms or ions in the crystal lattice, so the mass of the crystal remains the same.

No Change in Volume:
The volume of an ionic crystal is determined by the arrangement of the ions in the lattice. When a Frenkel defect occurs, the positions of the ions are not altered, only the location of some ions within the lattice. Therefore, the volume of the crystal remains the same.

Density Calculation:
Since both the mass and volume of the crystal remain unchanged when Frenkel defects are created, the density of the crystal also remains the same. Density is calculated by dividing the mass of the crystal by its volume. Since both the numerator and denominator are unchanged, the density remains constant.

Conclusion:
In conclusion, when Frenkel defects are created in an otherwise perfect ionic crystal, the density of the crystal remains the same. This is because Frenkel defects involve a redistribution of atoms within the lattice, without changing the number of atoms or the arrangement of the ions.