FOR BCC

i) Number of atoms per unit cell :-       (8 x 1/8) + 1 = 2.

ii) The co-ordination number :-       8, because there are eight unit cells surrounding a corner atom in BCC unit cell. Atom at the centre of the unit cell is nearest to the corner atom in unit cell.

iii) The packing fraction :-       2 x (4/3) PI (R)³ / (a)³
i.e.      2/1 x 4/3 x PI/1 x [(square root of 3) x a /4]³ x 1/ (a)³
i.e.      (square root of 3) x (PI / 8)
i.e.      0.68