## Unit – III A |

## Bravais Lattice Structures

### Unit Cell:

- A unit cell is the smallest structural repeating unit of crystalline solid.
- When unit cells of the same crystalline substance are repeated in space in all direction, a crystalline solid is formed.
- Unit cell is represented on paper by drawing lines connecting centres of constituent particles.
- Each point at the intersection of lines in the unit cell represents constituent particle viz. molecule or atom or ion. This point of intersection of lines in the unit cell is called lattice point.
- The collection of all the lattice points in the crystal having similar environment is called space lattice.

### Bravais Lattices:

- Let lengths of three edges of unit cell be a, b and c. Let α be the angle between side b and c. Let β be the angle between side a and c. Let γ be the angle between side a and b.
- French mathematician Bravais said that for different values of a, b, c and α, β, γ, maximum fourteen (14) structures are possible. These arrangements are called Bravais Lattices.

### Coordination Number:

- The coordination number of the constituent particle of the crystal lattice is the number of particles surrounding a single particle in a crystal lattice.
- More the coordination number more tightly the particles are packed in the crystal lattice.
- Coordination number is the measure of the hardness of the crystal.