Calculate the co`-` ordination number of an atom in `:`
`(i)` A primitive cubic unit cell,
`(ii)` A body`-` centred cubic unit cell.
`(iii)` A face `-` centred cubic unit cell.

Explanation

(i)In a primitive cubic unit cell, each atom has 6 equally spaced nearest neighbours .
(ii)In BCC, an atom at the body centre is surrounded by 8 nearest neighbours situated at the corners of the cube. Hence, C. N. =8.
(iii) In FCC, an atom is surrounded by 12 nearest neighbours (6 in its own plane, 3 above and 3 below). Hence , C.N. =12