Explanation: Atomic Packing Factor: In the crystal structure, atomic packing factor (APF) or packing efficiency or packing fraction is the volume of atoms in a unit cell divided by the volume of the unit cell. It is a dimensionless quantity and always less than unity No of atoms in b.c.c unit cell = 2 For BCC $a = \frac{{4r}}{{\sqrt 3 }}$ where a is the side of the cube and r is the atomic radius. Now APF will be $APF = \frac{{{N{atoms}}{V{atom}}}}{{{V{crystal}}}} = \frac{{2\left( {\frac{4}{3}} \right)\pi {r^3}}}{{{{\left( {a } \right)}^3}}}= \frac{{2\left( {\frac{4}{3}} \right)\pi {r^3}}}{{{{\left( {\frac{{4r}}{{\sqrt 3 }}} \right)}^3}}}$ $APF = \frac {\sqrt {3} \pi}{8}$ Unit Cell | Coordination No.