Assertion : Alkali halides do not show Frenkel defect
Reason : Ions of alkali halides have almost equal in size

Options

A. Assertion is false but reason is true
B. Assertion is true but reason is false
C. Both assertion and reason are true, but reason is not a true explanation for assertion
D. Both assertion and reason are true and reason is the correct explanation for assertion

Answer

To solve the question regarding the assertion and reason about alkali halides and Frenkel defects, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Alkali halides do not show Frenkel defect."
- We need to determine if this statement is true or false.

2. **Understanding the Reason**:
- The reason provided is that "Ions of alkali halides have almost equal in size."
- We need to analyze if this reason is true and whether it explains the assertion.

3. **Defining Frenkel Defect**:
- A Frenkel defect is a type of point defect in a crystal lattice where a cation is displaced from its original lattice site to an interstitial site, creating a vacancy at the original site.
- For a Frenkel defect to occur, the cation must be significantly smaller than the anion so that it can fit into the interstitial spaces of the lattice.

4. **Analyzing Alkali Halides**:
- Alkali halides (like NaCl, KBr, etc.) consist of cations (Li⁺, Na⁺, K⁺, etc.) and anions (Cl⁻, Br⁻, etc.).
- In alkali halides, the sizes of the cations and anions are relatively similar.

5. **Conclusion on the Assertion**:
- Since the cations in alkali halides are not significantly smaller than the anions, they do not fit into the interstitial spaces to create a Frenkel defect.
- Therefore, the assertion that "Alkali halides do not show Frenkel defect" is **true**.

6. **Conclusion on the Reason**:
- The reason that "Ions of alkali halides have almost equal in size" is also **true**.
- However, this reason does not fully explain why alkali halides do not show Frenkel defects, as the main factor is the inability of the cations to fit into the interstitial spaces due to their similar sizes.

7. **Final Evaluation**:
- Both the assertion and reason are true, but the reason does not serve as a correct explanation for the assertion.
- Thus, the correct answer is: Both assertion and reason are true, but the reason is not the correct explanation for the assertion.