The cordination number of a metal crystallising in a hexagonal close-packed structure is:

Options

A. 12
B. 4
C. 8
D. 6

Answer

To determine the coordination number of a metal crystallizing in a hexagonal close-packed (hcp) structure, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number of a crystal structure refers to the number of nearest neighbor atoms surrounding a central atom in the crystal lattice.

2. **Identifying the Structure**:
- The question specifies a hexagonal close-packed (hcp) structure. This is a common arrangement in metallic crystals.

3. **Analyzing the HCP Structure**:
- In the hcp structure, atoms are arranged in a way that maximizes packing efficiency. The arrangement consists of two layers of atoms (A and B) that alternate.

4. **Counting Nearest Neighbors**:
- In the hcp structure, each atom is surrounded by 12 other atoms. This includes:
- 6 atoms in the same layer (the equatorial plane),
- 3 atoms in the layer above,
- 3 atoms in the layer below.

5. **Conclusion**:
- Therefore, the coordination number for a metal crystallizing in a hexagonal close-packed structure is **12**.

### Final Answer:
The coordination number of a metal crystallizing in a hexagonal close-packed structure is **12**.

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