There are eight atoms situated at
the eight corners. The corner atoms touch each other. The lattice constant ‘a’
i.e. side of the unit cell and the radius of the atom ‘r’ on front face only is
shown in Fig. The ‘a’ and ‘r’ are related as
a = 2r ..........(1)
Face
Centred Cube (FCC): -
There are eight
corner atoms and one atom at the centre of each face. The corner atoms do not
touch each other but each corner atom touches the central atom of each face.
The front face arrangement is shown in fig.
From D ABC of this figure,
AC2 =
AB2 + BC2
\ (4r)2 = a2 + a2
or
8r2 = a2 Þ 
..........(2)
..........(2)
Body Centred
Cube (BCC): -
Fig.
(b) Relation between r and ‘a’ in FCC. (c) Relation between r and ‘a’ in BCC.
There are eight atoms at the corners
of the unit cell and one atom at the centre. The corner atoms do not touch each
other, but each corner atom touches the central atom. From Fig., the body
diagonal BD and planer diagonal BC are related as
BD2 =
BC2 + CD2 = (AB2 +
AC2) + CD2
As AB = AC = CD = a
\ (4r)2 = a2 + a2 + a2
or, 
..........(3)