Which has hcp crystal structure ?

Options

A. NaCl
B. CsCl
C. Zn
D. RbCl

Answer

To determine which substance has a hexagonal close-packed (hcp) crystal structure, we can analyze the options provided:

1. **Understanding HCP Structure**:
- HCP stands for Hexagonal Close Packing.
- It features a specific arrangement of atoms in a hexagonal lattice.
- The packing is characterized by an ABABAB... sequence, meaning that the layers alternate in a specific manner.

2. **Key Features of HCP**:
- **Coordination Number**: The coordination number for HCP is 12, indicating that each atom is in contact with 12 others.
- **Unit Cell Shape**: The unit cell of HCP is hexagonal.
- **Packing Efficiency**: The packing efficiency for HCP is 74%, meaning that 74% of the volume is occupied by the atoms.

3. **Analyzing the Options**:
- **Option 1: NaCl**: Sodium chloride has a face-centered cubic (fcc) structure, not hcp.
- **Option 2: CaCl2**: Calcium chloride does not adopt an hcp structure; it has a different arrangement.
- **Option 3: Zinc**: Zinc is known to crystallize in the hcp structure.
- **Option 4: RbCl**: Rubidium chloride also does not have an hcp structure; it typically forms a cubic lattice.

4. **Conclusion**:
- Based on the analysis, the substance that has an hcp crystal structure is **Zinc** (Option 3).

### Final Answer:
**Zinc has an hcp crystal structure.**