in which of the following structures coordination number for cations and anions in the packed structure will be same ?

Options

A. `Cl^(-)` ions form fcc lattice and `Na^(+)` ions occupy all octahedral voids of the unit cell
B. `Ca^(2+)` ions form fcc lattice and `F^(-)` ions occupy all the eight tetrahedral voids of the unit cell
C. `O^(2-)` ions form fcc lattice and `Na^(+)` ions occupy all the eight tetrahedral voids of the unit cell
D. `S^(2-)` ions form fcc lattice and `Zn^(2+)` ions go into alternate tetrahedral voids of the unit cell.

Explanation

To determine in which of the given structures the coordination number for cations and anions in the packed structure will be the same, we will analyze the coordination numbers for each of the mentioned compounds: NaCl, CaF2, Na2O, and ZnS.

### Step-by-Step Solution:

1. **Analyze NaCl (Sodium Chloride)**:
- In the NaCl crystal lattice, each Na⁺ ion is surrounded by 6 Cl⁻ ions.
- Conversely, each Cl⁻ ion is also surrounded by 6 Na⁺ ions.
- Therefore, the coordination number for both Na⁺ and Cl⁻ is **6**.
- **Conclusion**: Coordination number is the same for Na⁺ and Cl⁻.

2. **Analyze CaF2 (Calcium Fluoride)**:
- In the CaF2 structure, the Ca²⁺ ions form a face-centered cubic (FCC) lattice.
- The coordination number for Ca²⁺ is **4** (each Ca²⁺ is surrounded by 4 F⁻ ions).
- The F⁻ ions occupy all 8 tetrahedral voids, giving them a coordination number of **8**.
- **Conclusion**: Coordination numbers are different (Ca²⁺ = 4, F⁻ = 8).

3. **Analyze Na2O (Sodium Oxide)**:
- In Na2O, the O²⁻ ions form an FCC lattice.
- The Na⁺ ions occupy all 8 tetrahedral voids, resulting in a coordination number of **8** for Na⁺.
- The coordination number for O²⁻ is **4** (each O²⁻ is surrounded by 4 Na⁺ ions).
- **Conclusion**: Coordination numbers are different (Na⁺ = 8, O²⁻ = 4).

4. **Analyze ZnS (Zinc Sulfide)**:
- In ZnS, the S²⁻ ions form an FCC lattice.
- The Zn²⁺ ions occupy alternate tetrahedral voids, resulting in a coordination number of **4** for Zn²⁺.
- The coordination number for S²⁻ is also **4**.
- **Conclusion**: Coordination number is the same for Zn²⁺ and S²⁻.

### Final Answer:
The structures where the coordination number for cations and anions is the same are **NaCl** and **ZnS**.