1. The problem statement, all variables and given/known data The fraction of volume unoccupied in the unit cell of the body centered cubics lattice is? 2. Relevant equations 3. The attempt at a solution I got the volume occupied by the atoms as (8/3)πr3. I am not sure if it is right. I just assumed that all the atoms were of same element. I also don't know about the dimensions of the cube. Can somebody help me? It is from an old entrance exam question paper.