Unit cell:- Unit cells is the smallest portion of a crystal lattice which, when repeated in different directions generates the entire to lattice.

A unit cell is characterised by (i) its dimensions along the three edges, a, b and c. These edges may or may not be mutually perpendicular.

(ii) Angle between the edges α (betwen b and c), β (between a and c) and γ (between a and b).

Thus a unit cell is characterised by six parameters: a, b, c, α, β and γ unit cells can be broadly divided into two categories- primitive and centred unit cells.

(i) Primitive unit cells: When constituent particles are present only on the corner position of a unit cells, it is called as primitive unit cell.

(ii) Centred unit cells: When a unit cells contains one or more constituent particles present at positions other that corners in addition to those at corners it is called a centred unit cell. Centred unit cells are of three types:

(a) Body centred unit cells:- Such a unit cells contains one constituent particle (atom, molecules or ion) at this body centred besides to ones that are at corners.

(b) Face centred unit cells:- Such a unit cell containing one constituent particles present at the centre of its each face, besides the one that are at its corners.

(c) End centred unit cells:- In such a unit cell one constituent particles is present at the centre of any two opposite faces besides the one present at its corners.

In all there are seven types of primitive unit cells. They are cubic, tetragonal, orthorhombic, hexagonal, rhombohedral of trigonal monoclinic and triclinic.