Which of the following statements 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), we need to analyze the properties and characteristics of HCP. Here’s a step-by-step solution:

### Step 1: Understand the Structure of Hexagonal Close Packing
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 layers. The arrangement consists of alternating layers of spheres.

**Hint:** Remember that HCP consists of layers that alternate in arrangement, typically labeled as A and B.

### Step 2: Identify the Arrangement of Layers
In HCP:
- The first layer is designated as A.
- The second layer is different and is designated as B.
- The third layer aligns with the first layer (A).
- The fourth layer aligns with the second layer (B).

This creates an ABAB pattern.

**Hint:** Focus on how the layers are arranged and how they relate to each other.

### Step 3: Determine the Coordination Number
In HCP, each sphere is in contact with:
- 6 spheres in its own layer,
- 3 spheres in the layer above,
- 3 spheres in the layer below.

This gives a total coordination number of 12.

**Hint:** Recall the definition of coordination number and how it relates to the number of neighboring spheres.

### Step 4: Calculate Packing Efficiency
The packing efficiency of HCP is about 74%. This means that 74% of the volume is occupied by the spheres, while the remaining 26% is void space.

**Hint:** Packing efficiency is a measure of how closely the spheres are packed together.

### Step 5: Analyze the Statements
Now, we need to evaluate the provided statements about HCP to identify which one is not true:
1. Coordination number is 12.
2. It has 74% packing efficiency.
3. Tetrahedral voids of the second layer are covered by the spheres of the third layer.
4. The fourth layer is aligned with the first layer.

From our analysis:
- Statements 1, 2, and 3 are true.
- Statement 4 is false because the fourth layer is aligned with the second layer, not the first layer.

**Hint:** Carefully compare each statement with the characteristics of HCP that you have established.

### Conclusion
The statement that is not true about hexagonal close packing is: **"The fourth layer is aligned with the first layer."**