In crystalline solids, which of the following element of symmetry is not present ?

Options

A. Axis of symmetry
B. Angle of symmetry
C. Centre of symmetry
D. Plane of symmetry

Answer

To determine which element of symmetry is not present in crystalline solids, we can analyze the common types of symmetry found in these structures. Here’s a step-by-step solution:

### Step 1: Identify the Types of Symmetry
Crystalline solids can exhibit several types of symmetry, including:
- Axis of symmetry
- Center of symmetry
- Plane of symmetry
- Angle of symmetry

### Step 2: Analyze Each Type of Symmetry
1. **Axis of Symmetry**: This is a line through the crystal such that when the crystal is rotated around this line by a certain angle, it looks the same. Crystalline solids, such as cubic crystals, have multiple axes of symmetry.

2. **Center of Symmetry**: This is a point in the crystal such that for every point in the crystal, there is an identical point at an equal distance in the opposite direction. Cubic crystals have a center of symmetry.

3. **Plane of Symmetry**: This is a plane that divides the crystal into two mirror-image halves. Crystalline solids can possess multiple planes of symmetry.

4. **Angle of Symmetry**: This refers to the symmetry that would allow the crystal to look the same after being rotated by any angle. Crystalline solids do not have this type of symmetry because they only exhibit symmetry at specific angles (like 90 degrees for cubic crystals).

### Step 3: Conclusion
Based on the analysis, the element of symmetry that is not present in crystalline solids is the **angle of symmetry**.

### Final Answer
The correct answer is **angle of symmetry**.

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