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 determine which crystals have alternate tetrahedral voids occupied, we will analyze the structures of various compounds based on the information provided in the video transcript.

### Step-by-Step Solution:

1. **Understanding Tetrahedral Voids**:
- Tetrahedral voids are formed in a crystal lattice when four atoms (or ions) surround a central void. In a face-centered cubic (FCC) lattice, there are 8 tetrahedral voids per unit cell.

2. **Identifying the Compounds**:
- The compounds mentioned in the video are:
- Zinc Blende (ZnS)
- Calcium Fluoride (CaF₂)
- Sodium Chloride (NaCl)
- Sodium Oxide (Na₂O)

3. **Analyzing Each Compound**:
- **Zinc Blende (ZnS)**:
- In ZnS, the S²⁻ ions form an FCC lattice, and the Zn²⁺ ions occupy alternate tetrahedral voids. Therefore, ZnS has alternate tetrahedral voids occupied.

- **Calcium Fluoride (CaF₂)**:
- In CaF₂, the Ca²⁺ ions occupy the FCC lattice, while the F⁻ ions occupy all the tetrahedral voids. Thus, all tetrahedral voids are occupied, not alternate ones.

- **Sodium Chloride (NaCl)**:
- NaCl has a face-centered cubic structure where Na⁺ ions occupy octahedral voids, and Cl⁻ ions occupy tetrahedral voids. However, not all tetrahedral voids are occupied, and they are not occupied alternately.

- **Sodium Oxide (Na₂O)**:
- In Na₂O, the O²⁻ ions form an FCC lattice, and the Na⁺ ions occupy tetrahedral voids. Similar to CaF₂, all tetrahedral voids are occupied, not alternate ones.

4. **Conclusion**:
- The only compound from the list that has alternate tetrahedral voids occupied is **Zinc Blende (ZnS)**.

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

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