Crystal System: Unit Cell and Classification of Crystal System

Crystal lattice

A crystal lattice is an array of points representing atoms, ions, or molecules of a crystal, arranged at different sites in three-dimensional space.


A crystal is composed of atoms, ions, or molecules. In crystalline solids, these atoms, ions, or particles lie at definite positions in space. These positions are represented by points in a crystal. These points are called lattice points or lattice sites.

This setup of factors in a crystal is called crystal lattice or space lattice.

Unit Cell

The tiniest part of the crystal lattice has all the characteristic functions of the entire crystal and is called a unit cell.


It means that a unit cell of a crystal lattice is the tiniest block or geometric figure, where the whole crystal can be built up by duplicating it in three dimensions.

It reveals the structural properties of a provided crystal. The total information concerning the crystalline framework exists within a unit cell which duplicates itself in three dimensions to create a crystal.

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If we understand the specific arrangement of atoms in a unit cell, we as a matter of fact understand their position in the entire crystal. The measurable factors of a crystal lattice are reasoned from the size and shape of the unit cell.

There are three, unit cell lengths a, b, c, and three, unit cell anglesα, β, and γ. These six parameters of the unit cell are called unit cell dimensions or crystallographic factors.

Classification of Crystal System

A crystal system may be identified by the dimensions of its unit cell along its three edges or axes, a, b, c as well as three angles in between the axes α, β, and γ. There are seven crystal systems. These 7 crystal systems are called as follows:


1. Cubic system

In this system, all three axes are of equal size and all go to appropriate angles to one another.

2. Tetragonal system

In this system, two axes are of equal size, and the third axis is either shorter or larger than the other two. All angles are 90 °.

3. Orthorhombic Or Rhombic System

All three axes are of unequal size and all go to the right angle to each other.

4. Monoclinic System

All the three axes are of unequal size; two of these axes go to the right angle to each other while the third angle is greater than 90 °.

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5. Hexagonal System

In this system, two axes are of equivalent length and are in one plane making an angle of 120° with each other. The third axis which is different in size than the other two is at a right angle to these two axes.

6. Rhombohedral System or Trigonal System

All the three axes are of an equivalent size like the cubic system but the three angles are not equivalent as well as exist in between 90 ° and 120 °.

7. Triclinic System

All the three axes and the three angles are unequal as well as none of the angles is 90 °.