Question

For the crystal class, which one of the following statements is correct?
1. If a restraint is imposed on the growth of crystal, it will have an external symmetrical shape only 
2. The molecules are arranged in an irregular manner but in a repetitive pattern.
3. The shape will be symmetrical only if it possesses all the elements of symmetry
4. The shape will be symmetrical only about a plane if it divides the shape in two identical halves.
5.

Answer 1

Correct Answer - Option 4 : The shape will be symmetrical only about a plane if it divides the shape in two identical halves.

Concept:

The crystalline state is one in which the constituent atoms or molecules are arranged in a regular, repetitive, and symmetrical pattern, but a crystal will possess a symmetrical external shape only if no restraint is imposed during its growth.

• A shape is said to be symmetrical if it possesses one or more elements of symmetry.
• The elements of symmetry are planes, axes, and points of symmetry.
• A shape of low symmetry may possess only one plane of symmetry, whereas a highly symmetrical shape, such as a cube will contain several planes and axes of symmetry.
• A shape is said to be symmetrical about a plane if the plane divides the shape into either two identical halves or into two halves that are mirror images to each other.

There are seven crystal systems. Their symmetries are given below in the table,

Crystal System	Axial Relationships	Inter axial Angles
Cubic	a = b = c	α = β = γ = 90°
Hexagonal	a = b ≠ c	α = β = 90°, γ = 120°
Tetragonal	a = b ≠ c	α = β = γ = 90°
Rhombohedral (Trigonal)	a = b = c	α = β = γ ≠ 90°
Orthorhombic	α ≠ b ≠ c	α = β = γ = 90°
Monoclinic	a ≠ b ≠ c	α = γ = 90° ≠ β
Triclinic	a ≠ b ≠ c	α ≠ β ≠ γ ≠ 90