selenium

Selenium is a chemical element with atomic number 34, with the chemical symbol Se. Selenium occurs only rarely in the free state in nature. It is a nonmetal that is chemically related to sulfur and tellurium. It is toxic in large amounts, but trace amounts of it, forming the active center of certain enzymes such as glutathione peroxidase(GSH-Px), are necessary for the function of all cells in (probably) all animals. Selenium requirements in plants differ by species, with some plants apparently requiring none.

Isolated selenium occurs in several different forms, but the most stable of these is a dense purplish-gray semimetal (semiconductor) form that is structurally a trigonal polymer chain. It conducts electricity better in the light than in the dark, and is used in photocells (see allotropic section below). Selenium also exists in many nonconductive forms: a black glass-like substance, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter cousin sulfur.

Selenium is found in economic quantities partially replacing sulfur in sulfide ores such as pyrite. Minerals that are selenide or selenate compounds are also known, but all are rare.



The term "selenium" was found in the following pages:

Material database: IRMM - Institute for Reference Materials and Measurements - BCR-063R Skim milk powder (natural)
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Material database: National Metrology Institute of Japan (NMIJ) - NMIJ CRM 7303-a: Trace Elements in Lake Sediment
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Material database: LGC Ltd. - LGC7330 - Selenomethionine enriched with 76Se
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Link database: Selenium and Livestock: Metabolism, Toxicity and Deficiency
Material database: NIES - National Institute for Environmental Studies - CRM 28: Urban Aerosols
Material database: National Institute for Environmental Studies (NIES) - CRM 27: Typical Japanese Diet
Material database: IRMM - Institute for Reference Materials and Measurements - ERM-BD150: Skimmed milk powder
Directory of scientists: Bernhard Michalke
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Link database: Chemical Fact Sheet: Thallium
Material database: National Institute for Environmental Studies (NIES) - NIES CRM 18: Human Urine
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