The co-ordination number in 3D-hexagonal close packing is

Options

A. 8
B. 12
C. 6
D. 9

Answer

To determine the coordination number in 3D hexagonal close packing (hcp), we can follow these steps:

### Step-by-Step Solution:

1. **Understand Coordination Number**:
- The coordination number is defined as the number of atoms or ions that surround a central atom in a crystal structure.

2. **Identify the Structure of Hexagonal Close Packing (hcp)**:
- In hcp, the atoms are arranged in layers. The arrangement follows an "ABAB" stacking sequence, where 'A' and 'B' represent different layers.

3. **Visualize the Arrangement**:
- In the first layer (A), there are six atoms surrounding a central atom. This can be visualized as a hexagon where the central atom is in the middle and the six surrounding atoms are at the vertices of the hexagon.

4. **Consider the Second Layer (B)**:
- The second layer (B) sits directly above the first layer (A). Each atom in the B layer occupies a position above the gaps between the atoms in the A layer.

5. **Count the Neighbors**:
- The central atom is surrounded by:
- 6 atoms from the first layer (A).
- 6 atoms from the second layer (B) directly above it.
- Thus, the total number of atoms surrounding the central atom is \(6 + 6 = 12\).

6. **Conclusion**:
- Therefore, the coordination number in 3D hexagonal close packing is **12**.

### Final Answer:
The coordination number in 3D hexagonal close packing is **12**.

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