Start by taking four atoms and arranging them in a square. Then take four more atoms and arrange them in a square. Then put the first square on the second square to form a cube with eight atoms, one at each corner. This structure is the simple cubic crystal structure. It turns out that only the metal Polonium (Po) has this crystal structure. The reason this crystal structure is so rare is that packing atoms in this way does not lead to a very high packing density. In the next section, we will add one atom to the simple crystal structure and produce a crystal structure that is much more common.