Selenium is a nonmetal in Group VIA (Group 16) and the 4th period. Other elements in the group are oxygen (O), sulfur (S), tellurium (Te), and polonium (Po).
Selenium was discovered by Berzelius in 1817. Since the element was closely associated with tellurium, which is named for the earth, Berzelius named selenium for the moon, and the symbol is an abbreviation of the name.
The element, a nonmetal, is a member of the sulfur family and resembles sulfur both in its various forms and in its compounds.
Selenium exists in at least six allotropic forms. Three red forms have Se8 rings (like S8). A gray form has helical, polymeric chains. The normal commerical form, black vitreous Se, has an irregular structure.
Selenium is found in a few rare minerals, such as crooksite and clausthalite. In years past it has been obtained from flue dusts remaining from processing copper sulfide ores, but the anode muds from electrolytic copper refineries now provide the source of most of the world's selenium. Selenium is recovered by roasting the muds with soda or sulfuric acid, or by smelting them with soda and niter.
Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red; crystalline hexagonal selenium is a metallic gray.
Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells.
Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor, and is finding many uses in electronic and solid state applications.
It is used in Xerography for reproducing and copying documents. It is used by the glass industry to decolorize glass and to make ruby colored glasses and enamels. It is also used as a photographic toner and as an additive to stainless steel.
Elemental selenium has been said to be practically nontoxic; however, hydrogen selenide and other selenium compounds are extremely toxic, and resemble arsenic compounds in their physiological reactions. Nonetheless, selenium compounds are said to be important nutrients in trace amounts, are used in shampoos (SeS), and in veterinary medicines (Na2SeO3).
Selenium occurs in some soils in amounts sufficient to produce serious effects on animals feeding on plants, such as locoweed, grown in such soils.