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 nighbours. Hence, C.N. = 6.
(ii) In BCC, an atom at the body centre is surrounded by 8 nearest neighbours situted at the corners of the cube. Hence , C.N. = 8 ( Refer to fig 1.12)
(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