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 (A) and reason (R) given about the NaCl structure.

**Assertion (A):** In NaCl structure, the interionic distance is a/2 (a = Unit cell edge length).

**Reason (R):** NaCl forms a face-centered cubic (FCC) unit cell.

### Step-by-Step Solution:

1. **Understanding NaCl Structure:**
- NaCl (sodium chloride) crystallizes in a face-centered cubic (FCC) lattice structure. In this structure, chloride ions (Cl⁻) occupy the corners and face centers of the cubic unit cell.

2. **Identifying the Positions of Ions:**
- In the FCC unit cell, the Cl⁻ ions are located at the corners of the cube and at the centers of each face. The sodium ions (Na⁺) occupy the octahedral voids, which are located at the center of the cube and at the edge centers.

3. **Calculating the Interionic Distance:**
- The edge length of the unit cell is denoted as 'a'.
- The distance between the Na⁺ and Cl⁻ ions can be determined by considering the geometry of the FCC structure.
- The interionic distance (d) is the distance between the center of the Na⁺ ion and the center of the Cl⁻ ion.

4. **Using the Geometry of the FCC:**
- In an FCC unit cell, the distance between the centers of the Na⁺ and Cl⁻ ions can be calculated as follows:
- The Na⁺ ion is located at the center of the octahedral void, which is at a distance of a/2 from the Cl⁻ ions located at the corners.
- Therefore, the interionic distance (d) is equal to a/2.

5. **Conclusion on Assertion:**
- The assertion that the interionic distance in NaCl structure is a/2 is correct.

6. **Conclusion on Reason:**
- The reason that NaCl forms a face-centered cubic unit cell is also correct.

7. **Final Evaluation:**
- Both the assertion and the reason are true, and the reason is the correct explanation for the assertion.

### Final Answer:
Both the assertion (A) and reason (R) are true, and the reason is the correct explanation for the assertion.

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