B) The unit cell for Cr2O3 has hexagonal symmetry with lattice parameters a = 0.4961 nm and c =1.360 nm. If the density of this material is 5.22 g/cm", calculate its atomic packing factor. For this computation assume ionic radii of 0.062 nm and 0.140 nm, respectively, for Cr3+ and 02-. (Acr=52 g/mol, Ao=16 g/mol)