How many number octahedral voids are effectively inside an HCP unit cell

Answer

To determine how many octahedral voids are effectively inside a hexagonal close-packed (HCP) unit cell, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the HCP Structure**:
- The hexagonal close packing (HCP) is a type of crystal structure where atoms are closely packed. It consists of two hexagonal layers and one triangular layer in between.

2. **Identify the Number of Atoms in HCP**:
- In an HCP unit cell, there are a total of 6 effective atoms. This is similar to the face-centered cubic (FCC) structure, which has 4 effective atoms.

3. **Determine the Number of Octahedral Voids**:
- The number of octahedral voids in a crystal structure is equal to the number of atoms present in that structure. Therefore, in an HCP unit cell, the number of octahedral voids is also 6.

4. **Determine the Number of Tetrahedral Voids**:
- For reference, the number of tetrahedral voids in an HCP unit cell is twice the number of octahedral voids. Hence, there are 12 tetrahedral voids in an HCP unit cell.

5. **Conclusion**:
- The effective number of octahedral voids present in an HCP unit cell is 6.

### Final Answer:
The number of octahedral voids effectively inside an HCP unit cell is **6**.

---