Which of the following statements is/are correct about hexagonal close packing ?
1. The coordination number is 8
2. It is ABAB type packing in which third layer is aligned with the first layer
3. Be, Mg, Mo etc. are found to have hcp structure
4. In hcp, atoms occupy 74% of the available space

Options

A. 2,3
B. 3,4
C. 2,3,4
D. 1,2,3

Answer

To solve the question regarding hexagonal close packing (hcp), let's analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement 1: The coordination number is 8.**
- In hexagonal close packing, each atom is surrounded by 12 other atoms. Therefore, the coordination number is 12, not 8.
- **Conclusion:** This statement is **incorrect**.

2. **Statement 2: It is ABAB type packing in which the third layer is aligned with the first layer.**
- In hcp, the arrangement of atoms is indeed ABAB type, where the first layer (A) is aligned with the third layer (A) and the second layer (B) is aligned with the fourth layer (B).
- **Conclusion:** This statement is **correct**.

3. **Statement 3: Be, Mg, Mo etc. are found to have hcp structure.**
- Beryllium (Be), Magnesium (Mg), and Molybdenum (Mo) are known to crystallize in the hexagonal close packing structure.
- **Conclusion:** This statement is **correct**.

4. **Statement 4: In hcp, atoms occupy 74% of the available space.**
- The packing efficiency of hexagonal close packing is indeed 74%, meaning that 74% of the volume is occupied by the atoms, while the remaining 26% is empty space.
- **Conclusion:** This statement is **correct**.

### Final Conclusion:
The correct statements about hexagonal close packing are **2, 3, and 4**.