In hcp arrangement the coordination number is

Options

A. 6
B. 12
C. 8
D. 10

Answer

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

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
The coordination number is defined as the total number of nearest neighbor atoms surrounding a given atom in a crystal structure.

2. **Visualizing the hcp Structure**:
In the hcp arrangement, atoms are arranged in a hexagonal pattern. Each atom is surrounded by other atoms in a specific geometric configuration.

3. **Identifying Nearest Neighbors**:
For an atom in the hcp structure:
- There are 6 atoms in the same plane (the basal plane).
- There are 3 atoms in the plane above (the upper plane).
- There are 3 atoms in the plane below (the lower plane).

4. **Calculating the Total Coordination Number**:
To find the total coordination number, we sum the number of nearest neighbors:
- Neighbors in the same plane: 6
- Neighbors in the upper plane: 3
- Neighbors in the lower plane: 3

Therefore, the total coordination number = 6 (same plane) + 3 (upper plane) + 3 (lower plane) = 12.

5. **Conclusion**:
Thus, the coordination number in the hcp arrangement is 12.

### Final Answer:
The coordination number in hcp arrangement is **12**.

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