One cannot use the gas volume, 22.4 l, which is for a gas at STP. Plutonium is a solid at STP conditions.
Normally one would use a lattice parameter and the molecular mass (in amu) would need to be converted to kg is one is using SI or MKS units, or grams if one uses cgs. The lattice parameter (interatomic spacing) would have to be in the appropriate units.
Density could be in kg/m3 or g/cm3.