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. **Understanding Coordination Number**:
The coordination number is defined as the number of nearest neighbor atoms that surround a central atom in a crystal structure.

2. **Visualizing the HCP Structure**:
In a hexagonal close-packed structure, atoms are arranged in layers. The arrangement consists of alternating layers of atoms.

3. **Identifying the Central Atom**:
Consider a central atom located in one of the layers (let’s say layer B).

4. **Counting Nearest Neighbors**:
- **Above Layer**: The central atom in layer B is surrounded by 6 atoms in the layer directly above it (layer A).
- **Below Layer**: The central atom is also surrounded by 6 atoms in the layer directly below it (another layer B).
- **Adjacent Atoms**: Additionally, there are 3 atoms from the layer directly below (layer A) that are also in contact with the central atom.

5. **Total Coordination Number Calculation**:
- From the layer above: 6 atoms
- From the layer below: 6 atoms
- From the adjacent layer: 0 atoms (since they are already counted in the above and below layers)
- Therefore, the total coordination number = 6 (above) + 6 (below) = 12.

6. **Conclusion**:
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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