Definition of Unit Cell
The smallest group of atoms which has the overall symmetry of a crystal, and from which the entire lattice can be built up by repetition in three dimensions is termed as Unit Cell.
Crystalline solids exhibit a regular and repeating pattern of constituent particles.
The diagrammatic representation of the three-dimensional arrangement of constituent particles in a crystal, in which each particle is depicted as a point in space is known as crystal lattice.
What is a Lattice?
A lattice is a framework, resembling a three-dimensional, periodic array of points, on which a crystal is built. In 1850, M. A. Bravais showed that identical points can be arranged spatially to produce 14 types of regular patterns. These 14 space lattices are known as Bravais lattices.
The crystal lattice of a solid can be described in terms of its unit cell. A crystal lattice is made up of a very large number of unit cells where every lattice point is occupied by one constituent particle. The unit cell can be seen as a three-dimensional structure containing one or more atoms.
We can determine the volume of this unit cell with the knowledge of the dimensions of the unit cell. For example: if we have a unit cell of an edge “a”, the volume of the unit cell can be given as “a3”. The density of a unit cell is given as the ratio of mass and volume of the unit cell. The mass of a unit cell is equal to the product of the number of atoms in a unit cell and the mass of each atom in a unit cell.
Mass of unit cell = number of atoms in unit cell × mass of each atom = z × m
Where, z = number of atoms in the unit cell,
m = Mass of each atom
Mass of an atom can be given with the help of Avogadro number and molar mass as:
Where, M = molar mass
NA = Avogadro’s number
Volume of unit cell, V = a3
Thus, with the knowledge of number of atoms in a unit cell, edge length and molar mass we can determine the density of a unit cell.
A general expression for the density of unit cells for various cases has been derived below:
1. Primitive unit cell: In a primitive unit cell, the number of atoms in a unit cell, z is equal to one. Hence, density is given as:
2. Body-centered cubic unit cell: In body-centered cubic unit cell, the number of atoms in a unit cell, z is equal to two. Hence, density is given as:
3. Face-centered cubic unit cell: In face-centered cubic unit cell, the number of atoms in a unit cell, z is equal to four. Hence, density of unit cell is given as:
Frequently Asked Questions – FAQs
What is the volume of a unit cell?
The unit cell volume (V) is equal to the cubed cell-edge length (a). In a face-centered cubic structure, there would be four atoms per unit cell and the nickel density in this structure would be four times as high.
What is the formula for the density of any crystal?
Density of a unit cell is given as the ratio of mass and volume of the unit cell. The mass of a unit cell is equal to the product of the number of atoms in a unit cell and the mass of each atom in a unit cell.
What is difference between primitive cell and unit cell?
A unit cell is the smallest group of atoms which has the overall symmetry of a crystal, and from which the entire lattice can be built up by repetition in three dimensions. A primitive cell in chemistry is the smallest possible the unit cell of a lattice, having lattice points at each of its eight vertices only
What are the types of primitive unit cell?
There are seven primitive systems of crystals; cubic, tetragonal, orthorhombic, hexagonal, monoclinic, rhombohedral and triclinic. The structure of their crystallographic axes and angles differs between them.
What is unit cell in a solid state?
The most fundamental and lowest volume-consuming repeated form of any solid is a unit cell. It is used to visually simplify the solids in which crystalline patterns organise themselves. The network is called a lattice as the unit cell repeats itself.