Crystalline Solids and Symmetry Elements

Crystalline solids are characterized by a highly ordered arrangement of atoms or molecules, forming a regular repeating pattern called a crystal lattice. This periodic arrangement gives rise to various symmetry elements that can be found within the crystal structure. These symmetry elements help in understanding and classifying different types of crystals.

Symmetry Elements

Symmetry elements are specific features or characteristics that remain unchanged under certain operations. In the case of crystalline solids, the three main symmetry elements are:

1. Axis of Symmetry: An axis of symmetry is an imaginary line around which the crystal can be rotated by a certain angle without changing its appearance. The common types of axis of symmetry are 2-fold, 3-fold, 4-fold, and 6-fold axes. Each axis of symmetry represents a rotation operation.

2. Angle of Symmetry: An angle of symmetry is a line that divides an angle into two equal parts, such that if the crystal is rotated by that angle around the line, it will appear unchanged.

3. Centre of Symmetry: A centre of symmetry is a point within the crystal where an imaginary line can be drawn through it, and any atom or molecule on one side of the line is mirrored or reflected to the other side. In other words, if the crystal is inverted through the centre of symmetry, it will appear unchanged.

Determining the Absence of Symmetry Element

In the given question, we need to identify the symmetry element that is not present in crystalline solids. Let's analyze each option:

1. Axis of Symmetry: Axis of symmetry is commonly found in crystalline solids and represents the rotational symmetry of the crystal. It can be present in various forms.

2. Angle of Symmetry: Angle of symmetry is also a common symmetry element found in crystalline solids. It helps in dividing an angle into equal parts.

3. Centre of Symmetry: Centre of symmetry is a crucial symmetry element that is often found in crystalline solids. It represents the reflection symmetry of the crystal.

The Absent Symmetry Element

4. Plane of Symmetry: The plane of symmetry is the symmetry element that is not present in all crystalline solids. A plane of symmetry is an imaginary plane that divides the crystal into two identical halves, such that one half is the mirror image of the other. This symmetry element is absent in certain crystal structures.

Therefore, the correct answer is option (4) - plane of symmetry.

Conclusion

In conclusion, while axis of symmetry, angle of symmetry, and centre of symmetry are commonly found in crystalline solids, the plane of symmetry is not always present. The presence or absence of different symmetry elements helps in classifying and understanding the crystal structures.