Assertion (A) Both Frenkel and Schottky defects are stoichiometric defects.
Reason (R ) Both defects change the density of the crystalline solid.

Options

A. If both Assertion and Reason are true and Reason is correct explanation of Assertion.
B. If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. If Assertion is true but Reason is false.
D. If both Assertion and Reason and false.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that both Frenkel and Schottky defects are stoichiometric defects.
- **Stoichiometric Defects**: These are defects where the overall stoichiometry (the ratio of cations to anions) of the solid remains unchanged.
- **Frenkel Defect**: Involves the displacement of an ion from its lattice position to an interstitial site, maintaining the overall stoichiometry.
- **Schottky Defect**: Involves the creation of vacancies for both cations and anions, but the ratio of cations to anions remains the same, thus maintaining stoichiometry.
- **Conclusion**: Both defects are indeed stoichiometric defects. Therefore, the assertion is **True**.

2. **Understanding the Reason (R)**:
- The reason states that both defects change the density of the crystalline solid.
- **Effect on Density**:
- **Schottky Defect**: Creates vacancies for both cations and anions, which decreases the mass of the crystal while the volume remains the same, leading to a decrease in density.
- **Frenkel Defect**: Although it creates a vacancy, the ion that is displaced occupies an interstitial site, resulting in no net loss of ions. Therefore, the mass remains the same and the density does not change.
- **Conclusion**: The statement that both defects change the density is **False** because only the Schottky defect changes the density, while the Frenkel defect does not.

3. **Final Conclusion**:
- Assertion (A) is True.
- Reason (R) is False.
- Therefore, the correct answer is that the assertion is true, but the reason is false.