In a NaCl structure, if positions of Na atoms and Cl - atoms are interchanged, then in the new unit cell

Options

A. Na atoms is present at body centre
B. Cl atom is present at face centre
C. Na atom is present in tetrahedral voids
D. Cl atoms is present in octahedral voids

Answer

To solve the problem of what happens when the positions of Na+ and Cl- ions are interchanged in a NaCl structure, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the NaCl Structure**:
- In a typical NaCl structure, sodium ions (Na+) are located at the corners and the face centers of the cubic unit cell, while chloride ions (Cl-) occupy the octahedral voids.

2. **Identifying the Positions**:
- In the NaCl unit cell:
- Na+ ions are at the face-centered positions (FCC).
- Cl- ions are at the octahedral voids.

3. **Interchanging Positions**:
- If we interchange the positions of Na+ and Cl-, then:
- Cl- ions will now occupy the face-centered positions.
- Na+ ions will occupy the octahedral voids.

4. **Analyzing the New Structure**:
- After the interchange:
- The Cl- ions will be at the corners and face centers of the unit cell.
- The Na+ ions will be in the octahedral voids.

5. **Resulting Structure**:
- The new structure formed will not be stable because the sizes of Na+ and Cl- ions are different. Na+ is smaller than Cl-, and placing the larger Cl- ions in positions meant for smaller Na+ ions will lead to instability.

6. **Conclusion**:
- The new arrangement will not represent a stable ionic solid and will likely lead to a distortion of the crystal lattice.

### Final Answer:
When the positions of Na+ and Cl- ions are interchanged in a NaCl structure, the resulting unit cell will not be stable due to the size mismatch and will lead to a distorted structure.