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

Options

A. The coordination number is 12
B. It has 74% packing efficiency .
C. Tetrahedral voids of the second layer are covered by the spheres of the third layer
D. In this arrangement spheres of the fourth layer are exactly aligned with those of the first layer .

Explanation

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

### Step 1: Understanding Hexagonal Close Packing (HCP)
Hexagonal close packing is a way of arranging spheres (atoms, ions, or molecules) in a three-dimensional space. In HCP, the spheres are packed in a manner that maximizes the density of the arrangement.

### Step 2: Analyzing Each Statement
1. **Statement A**: "The coordination number is 12."
- **Analysis**: In HCP, each sphere is in contact with 12 other spheres. Therefore, this statement is true.

2. **Statement B**: "It has a 74% packing efficiency."
- **Analysis**: The packing efficiency of HCP is indeed 74%, meaning that 74% of the volume is occupied by the spheres. This statement is also true.

3. **Statement C**: "Tetrahedral voids of the second layer are covered by the sphere of the third layer."
- **Analysis**: In HCP, the arrangement allows the spheres in the third layer to cover the tetrahedral voids created by the second layer. This statement is true.

4. **Statement D**: "In this arrangement, spheres of the fourth layer are exactly aligned with those of the first layer."
- **Analysis**: In HCP, the spheres of the fourth layer are not exactly aligned with those of the first layer. Instead, they are offset due to the arrangement of layers (A-B-A). Therefore, this statement is false.

### Conclusion
The statement that is not true about hexagonal close packing is:
**D: In this arrangement, spheres of the fourth layer are exactly aligned with those of the first layer.**