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 using two types of layers, denoted as A and B.

2. **Layer Arrangement**:
- In the HCP structure, the first layer (A) is followed by a second layer (B). The third layer is again an A layer, and the fourth layer is a B layer, continuing this alternating pattern.

3. **Coordination Number Definition**:
- The coordination number is defined as the number of nearest neighbor atoms surrounding a central atom in a crystal structure.

4. **Analyzing the Layers**:
- In the HCP arrangement, each atom in the A layer is surrounded by atoms in the B layer. Specifically, each atom in the HCP structure touches 12 other atoms: 6 in the same layer, 3 in the layer above, and 3 in the layer below.

5. **Calculating the Coordination Number**:
- Since each atom is in contact with 12 other atoms, we conclude that the coordination number for the HCP structure is 12.

6. **Conclusion**:
- Therefore, 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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