which of the following statemets is not true about the hexagonal close packing ?

Options

A. In hcp, atoms occupy 74% the available space
B. It is AB AB type packing in which third layer is aligned with the first layer
C. Be, Mg, Mo etc. are found to have hcp structure
D. The coordination number is 6

Explanation

To determine which statement is not true about hexagonal close packing (HCP), let's analyze each of the provided options step by step.

### Step-by-Step Solution:

1. **Understanding Hexagonal Close Packing (HCP)**:
- HCP is a type of arrangement of atoms in a crystal structure where the atoms are packed closely together to maximize density.

2. **Analyzing the Statements**:
- **Statement 1**: "In HCP, atoms occupy 74% of available space."
- This statement is true. In hexagonal close packing, the packing efficiency is indeed 74%.

- **Statement 2**: "It is the ABAB type packing in which the third layer is aligned with the first layer."
- This statement is also true. In HCP, the arrangement alternates layers (A and B), and the third layer aligns with the first layer.

- **Statement 3**: "Beryllium, Magnesium, Molybdenum, etc. are found to have an HCP structure."
- This statement is true. These metals are known to crystallize in the hexagonal close-packed structure.

- **Statement 4**: "The coordination number is 6."
- This statement is false. The coordination number for hexagonal close packing is actually 12, not 6.

3. **Conclusion**:
- The statement that is not true about hexagonal close packing is the fourth statement: "The coordination number is 6."

### Final Answer:
The statement that is not true about hexagonal close packing is: **"The coordination number is 6."**

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