Types Of Unit Cells

A solid state of the matter is characterized by strong intermolecular forces and is a highly ordered structure as compared to the gaseous or liquid phase. Solids can be amorphous or crystalline. Amorphous solids are called supercooled liquids. In this state, molecules or atoms of a compound are arranged but albeit in a random manner. The other form of solid is crystalline and is most commonly observed in nature. Almost all metallic and ionic compounds do exist in crystalline form. Crystalline solids are characterized as highly ordered structures where molecular or atomic components are arranged in a specific pattern with strong intermolecular forces. This ordered pattern, symmetry in structure, and degree of packaging dictate other physical and chemical properties of solid compounds. Such arrangements achieve a minimum standard of intermolecular energy and hence are a more stable form of matter. 

Crystalline solid and lattice structure:

Crystalline solids show almost every time a specific lattice arrangement of component molecules or atoms. A lattice can be stated as an ordered repeating arrangement of particles in a three-dimensional space. Crystal could be made up of any of these seven types of lattice systems. These different lattice systems are defined by their axial pattern and constituent repeating units. These repeating units are basic structural units of crystalline solids and are called unit cells. Seven different lattice systems are the result of a different combination of unit cells and specific axial characteristics. 

Unit cells and types of unit cells:

As mentioned above, a unit cell is the basic structural repeating unit of a crystal. A unit cell is the characteristic arrangement of molecules and atoms of a compound. These are arranged to utilize the maximum space available and minimize the intermolecular energy of a crystal lattice. Locations of molecules or atoms on a unit cell are represented by dots known as lattice points. In practical reference, lattice points or coordinates only represent the centre of a participating particle. These points are joined together to form a diagrammatic representation of a unit cell. Most of the unit cells are cubic with some or more different characteristic features. These unit cells are repeated in a 3D space to form a lattice structure.

The arrangement of molecules or atoms in a unit cell and symmetry observed in a unit cell is the microscopic replica of lattice arrangement and symmetry. This arrangement also gives information on the number of molecules or atoms a particular molecule or atom makes contact with. This is called a coordination number. 

There is a total of 14 different types of unit cells. But most studied types are four, and these are named as-.

• Simple cubic unit cell
• Base centred unit cell
• Body-centred unit cell
• Face centred unit cell

Let’s see some details of a few types of these unit cells.

• Simple cubic centred unit cell:

In a simple cubic centred unit cell, each vertex of a cube represents the centre of one particle (molecule or atom). This means each particle contributes its 1/8th part to one unit cell. In other words, we can say that one complete particle makes one unit cell. Here each particle at one vertex makes contact with a particle at the neighbouring vertex. Hence the edge of a unit cell of this type is equal to the sum of the atomic radii of two particles. This is also called the primitive cubic unit cell. This type of unit cell arrangement is quite inefficient in space usage and occupies around 52 per cent of the available space. This makes such an arrangement quite rare in nature. Indeed, only one metal crystallizes into a simple cubic unit cell type, and that metal is polonium. 

• Body-centred cubic unit:

Even in this arrangement, each corner of a cube represents the centre of a single particle like that of the simple cubic unit cell. But unlike primitive cubic unit cell types, the particles at vertexes do not touch each other. In addition, there is one more particle located at the centre of a cube in the body-centred cubic unit cell. The cell thus consists of a total of two atoms and has a coordination number of eight. This pattern occupies around 68 per cent of total available space and hence is more efficient than simple cubic type in packaging. Elements like potassium, barium, and chromium show this type of crystallization. 

• Face centred cubic unit cell:

This is the most efficient among all the three mentioned types of unit cells. In this type, also, like two others, the corners of the cube represent the centres of a single particle. But, in addition, each face of a cube also constitutes half a particle. Thus in effect, this type of unit cell comprises a total of four particles and has a coordination number of 12. This type of arrangement makes use of about 74 per cent of available space and shows the strongest & closest packaging. Hence it is also called cubic closest packaging. This type of arrangement is made with particles from three distinct layers, and each one repeats itself at the interval of three. Examples of the elements that crystallize in this type of unit cell type are aluminium, calcium, copper, etc.

Other than these three types, there is a hexagonal cubic cell unit and ten others. These different types of unit cells in various combinations make seven different types of lattice structures. The physical and chemical properties of the crystalline compound are indeed dependent on the arrangement of constituent molecules or atoms in a particular type of unit cell.

Conclusion:

The crystalline structure and lattice system of a solid crystalline compound are dependent on the type of unit cells. These unit cells are nothing but the repeating, basic structural units of a crystal. These dictate the physical and chemical properties of a compound. The arrangement is such that it should maximize intermolecular forces and space usage and minimizes intermolecular energy. The different types of unit cells are the result of the differential arrangement of component particles in layers and one above the other. There is a total of 14 types of unit cells, but four are the most studied. These are mentioned in the text. The face-centred cubic cell is most efficient in packaging, while the simple cubic cell is least efficient.