The structure of a crystalline solid, whether metal or not, is best described by considering its simplest repeating unit, which is referred to as its unit cell. The unit cell consists of lattice points that represent the locations of atoms or ions.
Unit Cell is the smallest part (portion) of a crystal lattice. It is the simplest repeating unit in a crystal structure. The entire lattice is generated by the repetition of the unit cell in different directions.
Types of Unit Cell
Listed below are types of a unit cell
1. Primitive Unit Cells
When the constituent particles occupy only the corner positions, it is known as Primitive Unit Cells.
2. Centred Unit Cells
When the constituent particles occupy other positions in addition to those at corners, it is known as Centred Unit Cell. There are 3 types of Centred Unit Cells:
- Body Centred: When the constituent particle is at the centre of the body, it is known as the Body Centred Unit cell.
- Face Centred: When the constituent particle is present at the centre of each face, it is known as the Face Centred Unit cell.
- End Centred: When the constituent particle is present at the centre of two opposite faces, it is known as an End Centred Unit cell.
Atoms in FCC unit cell
- An FCC unit cell contains four atoms: one-eighth of an atom at each of the eight corners (8X18 = 18 X 18 = 1 atom from the corners) and one-half of an atom on each of the six faces (6 x 12 = 36 x 12 = 3 atoms from the faces).
- The face-centred cubic (FCC) has a coordination number of 12 and contains 4 atoms per unit cell. The atoms in the corner are shared by 8 unit cells and contribute to 1/8 of themselves to the crystal lattice.
- Check out the video given below to know more about close packing and voids