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 the HCP Structure**:
- The hexagonal close-packed structure consists of layers of atoms arranged in a specific pattern. The arrangement is typically described as alternating layers of atoms.

2. **Layer Arrangement**:
- In HCP, the arrangement of atoms is often denoted as ABAB... where 'A' represents one type of layer and 'B' represents another. Each layer is closely packed, maximizing the number of neighboring atoms.

3. **Analyzing Coordination**:
- For a given atom in the HCP structure, we need to count how many atoms surround it.
- In one layer (let's say layer A), an atom is surrounded by 6 other atoms in that same layer.

4. **Counting Atoms in Adjacent Layers**:
- In addition to the 6 atoms in the same layer, the atom also has 3 atoms directly above it in the next layer (layer B) and 3 atoms directly below it in the previous layer (also layer A).
- Therefore, the total number of atoms surrounding a central atom in HCP is:
- 6 (from the same layer) + 3 (from the layer above) + 3 (from the layer below) = 12.

5. **Conclusion**:
- Thus, the coordination number of 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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