A : In NaCl structure, the interionic distance is a/2 (a = Unit cell edge length).
R : NaCl forms face centered cubic unit cell.

Options

A. both Assertion & Reason are true and the reason is the correct explanation of the assertion
B. both Assertion & Reason are true but the reason is not the correct explanation of the assertion
C. Assertion is true statement but Reason is false
D. both Assertion and Reason are false statements

Answer

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

**Assertion (A):** In NaCl structure, the interionic distance is a/2 (where a = Unit cell edge length).
**Reason (R):** NaCl forms a face-centered cubic (FCC) unit cell.

### Step-by-step Solution:

1. **Understanding the Structure of NaCl:**
- NaCl crystallizes in a face-centered cubic (FCC) structure. In this structure, chloride ions (Cl⁻) are located at the corners and face centers of the cube, while sodium ions (Na⁺) occupy the octahedral voids.

2. **Position of Ions in the Unit Cell:**
- In the FCC unit cell of NaCl:
- Cl⁻ ions are found at the corners (8 corners) and at the face centers (6 faces).
- Na⁺ ions are located at the body center and the edge centers of the cube.

3. **Calculating Interionic Distance:**
- The interionic distance refers to the distance between a Na⁺ ion and a Cl⁻ ion.
- The edge length of the unit cell is denoted as 'a'.
- The relationship between the edge length and the ionic radii can be expressed as:
\[ a = 2(r_{Na^+} + r_{Cl^-}) \]
- Since the distance between Na⁺ and Cl⁻ ions along the edge of the cube is half the edge length:
\[ \text{Interionic distance} = \frac{a}{2} \]
- This confirms the assertion that the interionic distance is indeed \( \frac{a}{2} \).

4. **Evaluating the Reason:**
- The reason states that NaCl forms a face-centered cubic unit cell, which is correct.
- However, while the reason is true, it does not directly explain why the interionic distance is \( \frac{a}{2} \). The interionic distance is derived from the arrangement of ions, not solely from the type of unit cell.

5. **Conclusion:**
- Both the assertion and reason are true, but the reason does not provide a correct explanation for the assertion.
- Therefore, the correct answer is that both statements are correct, but the reason does not explain the assertion.

### Final Answer:
- Both Assertion (A) and Reason (R) are true, but R does not provide a correct explanation for A.