Element:	Selenium
Symbol:	Se
Atomic number:	34
Atomic weight:	78.96(3)
Electron configuration:	[Ar] 3d10 4s2 4p4
Ground level:	3P2
Ionization potential:	9.7524 ev
Physical form:	Gray metallic crystal
Melting point:	220.5°C
Boiling point:	685°C
Critical temperature:	1493°C
Density:	4.81 g/cm3
Specific heat:	0.321 J/g•K
Valence	4, 6, -2

Origin of name: Gr. Selene, moon

Discovered by Berzelius in 1817, who found it associated with tellurium, named for the earth.

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 exists in several allotropic forms. Three are generally recognized, but as many as six have been claimed. 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, the most stable variety, is a metallic gray. Natural selenium contains six stable isotopes. Twenty nine other isotopes and isomers have been characterized.

The element is a member of the sulfur family and resembles sulfur both in its various forms and in its compounds. 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, but its use in this application has been decreasing in favor of certain organic compounds. 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 and is considered to be an essential trace element; however, hydrogen selenide and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some soils in amounts sufficient to produce serious effects on animals feeding on plants, such as locoweed, grown in such soils.

Selenium (99.5%) is priced at about $250/kg. It is also available in high-purity form at a cost of about $350/kg (99.999%).