Calculate the planar densities of atoms in the followingcases:
     Draw the exact atomicarrangements in each case and put down the dimensions.
     Cu            FCC                100                 110           111                 r (Å) = 1.267
     Al             FCC                100                 110           111                             1.432
     Au            FCC                100                 110           111                             1.442
     Ba             BCC               100                 110           111                             2.176
     Fe             BCC               100                 110           111                             1.241
     Nb            BCC               100                 110           111                             1.426
