Number of atoms per unit cell in a cubic unit cell
A crystal lattice is made up of very large number of unit cells and every lattice point is occupied by one constituent particle such as atoms or molecules. We generally consider three types of unit cell and for simplification only atoms are considered as constituent particles.
The empty space present between the constituents particles in a close packed structure
Primitive cubic unit cell
In primitive cubic unit cell, the atoms are present only at the corners. Every atom at the corner is shared among eight adjacent unit cells. There are four unit cells in the same layer and four in the upper (or lower) layer. Therefore a particular unit cell has only 1/8th of an atom.
Body centered cubic unit cell
A body centered cubic unit cell has atoms at each corners and an atom at the center of the structure.
Face centered cubic unit cell
A face centered cubic unit cell contains atoms at all the corners of the crystal lattice and at the center of all the faces of the cube. The atom present at the face-center is shared between two adjacent unit cells and only 12 of each atom belongs to a unit cell.