Assertion: Ion pairs are missing in Schottky defect.

Reason: Density of the crystal is lowered.

Assertion and Reason both are correct statements and reason is the correct explanation for assertion.

The correct answer is option 1. Assertion and Reason both are correct statements and reason is the correct explanation for assertion.

Schottky defect is a type of point defect in crystals, commonly observed in ionic compounds, where ions are missing from their regular lattice sites. Let's explain the assertion and reason provided:

Assertion: Ion pairs are missing in Schottky defect.

This statement is correct. In a Schottky defect, ions of similar sizes, usually cations and anions, are missing from their regular lattice positions. As a result, ion pairs are indeed missing, leading to a stoichiometric imbalance in the crystal structure.

Reason: Density of the crystal is lowered.

This statement is also correct. When ions are removed from the crystal lattice due to Schottky defect, the crystal's density decreases because there are fewer particles occupying the same volume. This reduction in density is a consequence of the missing ions, leading to a less compact arrangement of atoms or ions within the crystal lattice.

Explanation:

The absence of ion pairs in the crystal lattice due to Schottky defect disrupts the regular arrangement of ions in the crystal structure. This disruption results in a decrease in the crystal's density because there are fewer ions present per unit volume. Despite the decrease in density, the electrical neutrality of the crystal is maintained since equal numbers of cations and anions are removed.

The assertion and reason both provide accurate statements regarding Schottky defect. The absence of ion pairs due to Schottky defect indeed contributes to the decrease in the crystal's density. Therefore, the reason provides a valid explanation for the assertion.