Crystal Lattice and Unit Cell

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Q#6.3: Define (i) Crystal lattice (ii) Unit Cell.

Ans.(i) Crystal Lattice.

A particular three-dimensional arrangement of particles (atoms, ions, molecules) in a crystal is called a crystal lattice.

In a crystal lattice, particles are located at definite positions in space. These positions are represented by points in a crystal and are called lattice points or lattice sites. This arrangement is called a crystal lattice or space lattice. A crystal lattice actually shows the shape of a crystal.

(ii) Unit Cell.

The smallest geometrical portion of the crystal which is used to build up the whole crystal. OR

If is the smallest portion of a crystal lattice which includes all the positions of each type of particle in the crystal. For example, a cubic crystal lattice is composed of eight unit cells.

The unit cell is defined by axes a, b, c and angles α, β, ϒ.

These six parameters (a, b, c, α, β, ϒ) of the unit cell are called unit cell dimensions or crystallographic elements.

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