Metals are materials of one or more metallic elements (iron, titanium, aluminum), and can have relatively small amounts of nonmetallic elements (carbon, nitrogen). Atoms in metals are arranged in an orderly manner and are dense in comparison to ceramics and polymers. Mechanically, metals are stiff and strong. However, they are ductile and resistant to fracture. Metallic materials' electrons are nonlocalized (not bound to particular atoms). The nonlocalization allows metallic materials to possibly be good conductors of electricity and heat. Metals do not allow visible light to pass through. A polished surface will have a lustrous appearance. Some metals are magnetic. Face-centered cubic crystal structure, body-centered cubic crystal structure, hexagonal close-packed crystal structure are examples of different bonding structures in metallic materials.