Selenium, a chemical element. Selenium exists in four forms, one metallic and three nonmetallic. All four forms, and all selenium compounds, are poisonous. The metallic form, known as gray selenium, is the most stable form at ordinary temperatures. It is a fair conductor of heat and has a metallic luster. The metal conducts electricity more readily on exposure to light than in the dark; its electrical conductivity varies with the amount of illumination.

The three nonmetallic forms slowly change into gray selenium when heated. Monoclinic selenium is a dark red crystalline form that is chemically stable below about 338F. (170C.). Red amorphous selenium is a red powder. Vitreous selenium, a black glassy solid, is formed when molten selenium is rapidly cooled. It is a poor conductor of heat and electricity. Both vitreous selenium and red amorphous selenium are chemically stable below about 140F. (60 C.).

All forms except monoclinic selenium are commercially important. Most commercial selenium is used by the electronics industry in the manufacture of rectifiers and photoelectric cells. It is used as a decolorizer in the manufacture of clear glass and as a semiconductor in xerography. Selenium is also used in the manufacture of vulcanized rubber, pigments, red glass, deodorants, cortisone, and niacin.

Selenium forms many useful chemical compounds. Selenium oxychloride, an extremely corrosive liquid, is used as a solvent in paint and varnish removers. Selenium dioxide is used as an antioxidant in lubricating oils and in the manufacture of organic chemicals. Sodium selenate is used in medicine to treat certain animal diseases. Other selenium compounds are used in photographic toning baths, in insecticides and fungicides, and as solid lubricants in various aerospace devices.

Selenium was discovered by the Swedish chemist Jns J. Berzelius in 1817. The element is widely distributed in the earth's crust, but deposits are small. It sometimes occurs free (chemically uncombined) in nature, but is usually found combined with a metal (most commonly copper, lead, or silver) in compounds called selenides. It also occurs as an impurity in various sulfide minerals. In some semiarid regions certain plants, commonly called locoweeds, concentrate selenium from the soil and sometimes poison grazing animals that feed on them.

Most commercial selenium, either in the form of gray selenium or red amorphous selenium, is recovered as a by-product in the refining of copper sulfide ores. The leading selenium-producing countries are Japan, Canada, and the United States.

Symbol: Se. Atomic number: 34. Atomic weight: 78.96. Specific gravity (gray selenium): 4.79. Melting point (gray selenium): 422.6 F. (217 C.). Boiling point (gray selenium): 1,265 F. (685 C.). Selenium has six stable isotopes: Se-74, Se-76, Se-77, Se-78, Se-80, and Se-82. It belongs to Group VIA of the Periodic Table and can have a valence of -2, +4, or +6.