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 solve the question regarding the elements of symmetry present in crystalline solids, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Symmetry in Crystalline Solids**:
- In crystallography, symmetry elements are specific points, lines, or planes that describe the symmetrical properties of a crystal structure. The main types of symmetry elements include axes of symmetry, planes of symmetry, and centers of symmetry.

2. **Identify the Types of Symmetry Elements**:
- **Axis of Symmetry (Cn)**: An axis around which the crystal can be rotated by a certain angle (360°/n) and appear unchanged.
- **Plane of Symmetry**: A plane that divides the crystal into two mirror-image halves.
- **Center of Symmetry**: A point in the crystal where for any point at (x, y, z), there is an equivalent point at (-x, -y, -z).

3. **Analyze the Given Options**:
- The question asks which element of symmetry is not present in crystalline solids. We need to evaluate the common symmetry elements found in typical crystalline structures.

4. **Evaluate Each Symmetry Element**:
- **Axis of Symmetry**: Present in most crystalline solids (e.g., cubic crystals have multiple axes of symmetry).
- **Plane of Symmetry**: Also commonly present in crystalline solids (e.g., a cube can be divided into two equal halves).
- **Center of Symmetry**: Present in many crystalline solids, including cubic structures.

5. **Identify the Element Not Present**:
- The video transcript suggests that the angle of symmetry (which is not a standard term in crystallography) is likely the element that is not present in crystalline solids. Thus, we conclude that this is the correct answer.

### Final Answer:
The element of symmetry that is not present in crystalline solids is the **angle of symmetry**.

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