I posted this in the engineering section but no replies : ( ... Let us see if the physicists can help. 1. The problem statement, all variables and given/known data Given: Density (D) of Si is 2.328 g/cm^3. Si has diamond crystal structure. Find: Theoretical density of a single crystal of Si. 2. Relevant equations Theoretical density = m/V 3. The attempt at a solution I know mass = (number of atoms) x (mass of atom), in a diamond crystal structure there are: [(1/8) x 8] + [(1/2) x 6] + [(1) x 4] = 8 Si atoms/crystal structure Now calculating volume is my issue, do I need the lattice constant? If so, how does one calculate it? Or is it just V = (m x 8)/D = (m x 8)/2.328, where m = 28.1 amu converted to grams? However, if I do that, I think I'll end up with the given density. What is the difference between the given density and theoretical density? Please help. Thank you.