What is unit cell?The smallest part of the complete space lattice which on repetition again and again in all the possible direction results in the formation of crystal lattice/space lattice is called a unit cell.
Contribution of a lattice point at a particular position:location Contribution
Body center 1
Face center 1/2
Edge center 1/4
Classification of the unit cell?On the basis of the location of the lattice point within the unit cell, there may be two types of unit cells.
1st classification of unit cell
1.Primitive unit cellIn this type of unit cell lattice points are present only at corners.
For example SCC(Simple Cubic Unit Cell)
Lattice points at the corner
2.Non-Primitive unit cellIn this type of unit cell lattice points are present not only at corners but also at some other specific position. For example,
(a)BCC(Body center cubic unit cell)
Coordination number(Z)=Lattice point at corner+Lattice point at the body
(b)FCC(Face center cubic unit cell)
Coordination number(Z)=Lattice point at all corner+lattice point at each face
Where the coordination number(Z)is the total number of particles or atoms or lattice points per unit cell.
2nd classification of unit cell:On the basis of axial length in x,y, z-direction, and interfacial angles unit cell can be classified into 7 types which are called seven crystal system or seven crystal habits and these are Cubic, Tetragonal, Orthorhombic(Rhombic), Monoclinic, Triclinic, Hexagonal, Rhombohedral.
For brief understanding about the above seven crystals, follow the link below: