An element that forms positive ions when its compounds are in solution and whose oxides form hydroxides rather than acids with water. Approximately 75% of the elements are metals that occur in every group of the periodic table except VIIA and the noble gas groups. Most are crystalline solids with metallic luster, conductors of electricity, and have rather high chemical reactivity; many are quite hard and have high physical strength. They also readily form solutions (alloys) with other metals. The presence of very low percentage of other elements (not necessarily metals) profoundly affects the properties of many metals, e.g., carbon in iron. Mercury, cesium, and gallium are liquid at room temperature. Geologically, metals usually occur in the form of compounds that must be physically or chemically processed to yield the pure metal; common methods are application of heat (smelting), carbon reduction, electrolysis, and reduction with aluminum or magnesium. Metals fall into the following classifications, which are not mutually exclusive:
alkali metals
rare metals
alkaline-earth metals
rare-earth metals
transition metals
actinide metals
noble metals
light metals
platinum metals
heavy metals
The chemistry of metals, i.e., their behavior as atoms or ions, is a fundamental factor in electrochemical reactions, as well as in the metabolism of plants and animals, where many have essential nutrient and other biochemical functions. Among these are iron, copper, cobalt, potassium, and sodium, often in traces. Some metals are quite toxic, especially cadmium, mercury, lead, barium, chromium, and beryllium, both in elemental form and as compounds.See Alloy; Trace Element; Electroplating.
