Number of Atoms in a unit Cell
Primitive cubic unit cell has atoms only at its corner.
In all, since each cubic unit cell has 8 atoms on its corners, the total number of atoms in one unit cell is 8x1/8 = 1 atom.
Body-Centered Cubic Unit Cell
body-centred cubic (bcc)
unit cell has an atom at each of its corners and also one atom at its
body centre. Thus in a body-centered cubic
(bcc) unit cell:
(i) 8 corners 1/8 per corner atom = 8x1/8=1 atom
(ii) 1 body centre atom = 1X1= 1 atom
Total number of atoms per unit cell = 2 atoms
face-Centered Cubic Unit Cell
A face-centred cubic (fcc) unit cell contains atoms at all the
corners and at the centre of all the faces of the cube. Thus,
in a face-centred cubic (fcc) unit cell:
(i) 8 corners atoms 1/8 atom per unit cell= 8x1/8 = 1 atom
(ii) 6 face centered atoms x 1/2 atom per unit cell = 6x1/2 = 3 atoms
Total number of atoms per unit cell = 1+3=4 atoms