A majority of elements (about three-fourths) in the Periodic Table are metals, and of all the metals, more than two-thirds of them possess relatively simple cubic or hexagonal crystal structures. Metal atoms are bonded by a chemical bond known as the metallic bond. The metallic bonds are nondirectional and there is no constraint regarding bond angles (which is not true for covalently bonded materials). Even though a simple cubic system is relatively unknown in metals (or even among ele­ments), a-polonium exhibits a simple cubic crystal structure. The most common crystal structures in metals are discussed in detail in the following sections.