Chemical Element displaying certain properties, notably metallic luster, the capacity to lose Electrons and form a positive Ion, and the ability to conduct Heat and Electricity (see Conduction), by which it is normally distinguished from a nonmetal. The metals comprise
about two thirds of the known elements. Some elements, e.g., arsenic and
antimony, exhibit both metallic and nonmetallic properties, and are called
metalloids. Metals fall into groups in the Periodic Table determined by similar arrangements of the orbital electrons and a consequent
similarity in chemical properties. Such groups include the Alkali Metals (Group la in the periodic table), the Alkaline-Earth Metals (Group IIa), and
the Rare-Earth Metals (Lanthanide and Actinide series). Most metals other than
the alkali metals and the alkaline-earth metals are called transition metals
(see TRANSITION ELEMENTS). The oxidation (see Oxidation and Reduction) states, or Valence, of the metal ions vary from +1 for the alkali metals to +7 for some
transition metals. Chemically, the metals differ from the nonmetals in that they form
positive ions and basic oxides and hydroxides. Upon exposure to moist Air, a great many metals undergo corrosion, i.e., enter into a chemical reaction,
the Oxygen of the atmosphere uniting with the metal to form the oxide of the metal,
e.g., rust on exposed iron.