# Chemistry: Solid State: Properties of Crystalline Solids: Three-Dimensional Lattice and Unit Cells (For CBSE, ICSE, IAS, NET, NRA 2022)

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## Properties of Crystalline Solids

### Three-Dimensional Lattice and Unit Cells

- In three dimensions, the crystal structure of a solid is represented as a three-dimensional array of lattice points.
- The lattice points represent the positions of the constituent particles of the solid.
- In a crystal lattice a group of points can be used to generate the whole lattice. Such a group is called repeat unit or the unit cell of the crystal lattice.

- The unit cell is characterized by three distances along the three edges of the lattice (a, b and c) and the angles between them are represented as .
- The whole crystal lattice are represented by repeating the unit cell in the three directions.
- On the basis of the external appearance the known crystals can be classified into seven types. These are called crystal systems.
- The internal structure of crystal lattices contains only seven types of unit cells.
- The seven crystal systems with their unit distances and the angles are compiled in the table below:

- A unit cell containing a lattice point each at the centers of its faces in addition to the lattice points at the corners is called a face centered (F) unit cell.
- A unit cell with lattice points at the center of the unit cell and at the corners is called a body centered unit cell (I) .
- In some cases, in addition to the lattice points at the corners there are two lattice points at the centers of any two opposite faces. These are called as end centered (C) unit cells.
- The seven crystal systems when combined with these possibilities give rise to 14 lattice types which are called as Bravais lattices.