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 solve the question, we need to analyze the coordination numbers of cations and anions in different crystal structures. The coordination number refers to the number of nearest neighbors surrounding a particular ion in a crystal lattice.

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number is defined as the number of ions of opposite charge that surround a given ion in a crystal lattice. For example, in NaCl, each Na+ ion is surrounded by six Cl- ions.

2. **Analyzing NaCl Structure**:
- In NaCl, the chloride ions (Cl-) form a face-centered cubic (FCC) lattice. The sodium ions (Na+) occupy the octahedral voids in this lattice.
- Each Na+ ion is surrounded by six Cl- ions, and each Cl- ion is also surrounded by six Na+ ions.
- Therefore, the coordination number for both Na+ and Cl- in NaCl is 6.

3. **Examining Other Structures**:
- **Option B (CaF2)**: In calcium fluoride (CaF2), Ca2+ ions form an FCC lattice, while F- ions occupy the tetrahedral voids. Here, the coordination number for Ca2+ is 8, and for F-, it is 4. Thus, they are not equal.
- **Option C (Na2O)**: In sodium oxide (Na2O), O2- ions form an FCC lattice, and Na+ ions occupy the tetrahedral voids. The coordination number for O2- is 4, while for Na+, it is 8. Again, they are not equal.
- **Option D (SnO)**: In tin(II) oxide (SnO), Sn2+ ions occupy octahedral voids, while O2- ions form an FCC lattice. The coordination number for Sn2+ is 6, while for O2- is 4. They are not equal.

4. **Conclusion**:
- From the analysis, only in NaCl do the cations and anions have the same coordination number, which is 6. Therefore, the correct answer is **Option A (NaCl)**.

### Final Answer:
The correct structure where the coordination number for cations and anions is the same is **NaCl**.

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