I've calculates the density of aluminium using the debye theory. I found The atom density n = 2.24E28 m^-3 at 600k (assumed to be the same as at room temperature). Therefore the density is just the molecular mass m (m(grams) = (79amu / Avogadros number), multiplied by the atom density. Which I found to be 1000 kg m^-3. I'm asked to compared to this with a true value of 2700 Kg m^-3. My estimate just assumes all the mass is the atoms, but I'm not sure why the discrepancy is so large. I thought the majority of the mass of a solid was in the atoms.