As a rule, since atoms in a solid attract each other, the more tightly packed arrangements of atoms tend to be more common. (Loosely packed arrangements do occur, though, for example if the orbital hybridization demands certain bond angles.) Accordingly, the primitive cubic structure, with especially low atomic packing factor, is rare in nature, but is found in polonium. The "bcc" and "fcc", with their higher densities, are both quite common in nature. Examples of "bcc" include iron, chromium, tungsten, and niobium. Examples of "fcc" include aluminium, copper, gold and silver.