Hi Candidate,

Packing fraction is the fraction of volume in a crystal structure that is occupied by constituent particles. It is a dimensionless quantity and always less than unity.

Packing efficiency can be written as below, Packing efficiency = Volume occupied by 6 spheres ×100 / Total volume of unit cells

The body-centred cubic unit cell ( BCC ) has atoms at each corner of the cube and an atom at the centre of the structure. The packing efficiency of the BCC lattice is 68%.