In which of the following crystals alternate tetrahedral voids are occupied?

Options

A. `NaCl`
B. `ZnS`
C. `CaF_(2)`
D. `Na_(2)O`

Answer

To solve the question regarding which of the following crystals has alternate tetrahedral voids occupied, we will analyze the ionic crystals given in the options.

### Step-by-Step Solution:

1. **Understanding Ionic Crystals**:
- Ionic crystals consist of cations and anions. The arrangement of these ions creates voids in which other ions can reside.
- There are two types of voids in the crystal lattice: tetrahedral voids and octahedral voids.

2. **Identifying the Role of Ions**:
- In most ionic crystals, cations are typically found in the voids, while anions are packed in a specific arrangement (often cubic close packing).

3. **Analyzing Specific Crystals**:
- **NaCl**: In sodium chloride (NaCl), Na⁺ ions occupy all the octahedral voids, while Cl⁻ ions are in the FCC packing.
- **ZnS**: In zinc sulfide (ZnS), Zn²⁺ ions occupy alternate tetrahedral voids, while S²⁻ ions are in the FCC packing.
- **CaF₂**: In calcium fluoride (CaF₂), F⁻ ions occupy all tetrahedral voids, which is an exception.
- **Na₂O**: In sodium oxide (Na₂O), Na⁺ ions occupy all tetrahedral voids, and O²⁻ ions are in the FCC packing.

4. **Conclusion**:
- Among the given options, ZnS is the crystal where alternate tetrahedral voids are occupied by Zn²⁺ ions. Hence, the correct answer is **ZnS**.

### Final Answer:
**ZnS** is the crystal in which alternate tetrahedral voids are occupied.