Metals show metallic luster, are good conductors of electricity and heat, and are very malleable and ductile. Such properties are characteristic of bulk metals, although the definition of metal atoms or ions is not simple. Metallic elements form basic oxides or hydroxides in the +1 or +2 oxidation states, and become cations in aqueous acid solutions. All transition elements are metals, but main group elements are classified into metallic and nonmetallic elements. Germanium and polonium may also be included as metals. Boron, silicon, germanium, arsenic, antimony, selenium, and tellurium exhibit some metallic characteristics and they are sometimes called metalloids.
Thumbnail: Pure barium in protective argon gas atmosphere. (Public Domain; Matthias Zepper).