So far we have introduced two closest-packed lattice structures--hexagonal and ccp--along with their associated unit cell (hexagonal and fcc). There are also structure in which the atoms or ions are arranged close together but not as efficiently as hcp and ccp lattices. Figure 11.10 shows two other types of cubic unit cells. The body-centered cubic (bcc) unit cell has an atoms at each of the 8 corners and 1 in the middle of the cell (Figure 11.10b). All the group 1 metals and many transition metals have bcc unit cells. The simple cubic unit cell consists of just 8 corner atoms (Figure 11.10c). It is the least efficiently packed of the three structures in the cubic crystal system and quite rare among metals: only radioactive polonium (Po) forms a simple cubic unit cell.