Crystal lattice have a three dimensional array of repeating units. For example, in the NaCl crystal the repeating units are the Na⁺ and Cl⁻ ions. The Na⁺ and Cl⁻ ions are arranged alternately along the x, y and z axis. The smallest repeating unit of a cell is called unit cell. Crystal lattice have a definite geometry which is because of this rigid array of unit cells.
