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:

Gas chromatography for the separation of elemental species | EVISA's News
Link database: US FDA: Selenium Regulations
Directory of scientists: Przemyslaw Niedzielski
Directory of scientists: Doris Kuehnelt
Directory of scientists: Maria Ochsenkuehn-Petropoulou
Material database: IRMM - Institute for Reference Materials and Measurements - BCR-277R - Estuarine Sediment
Material database: Sigma-Aldrich - Matrix CRM - Soil, Sludge, Sediment: Trace Metals - Loamy Sand 4
Link database: Hazardous Substances Data Bank: Hydrogenselenide
Material database: National Institute of Standards and Technology (NIST) - SRM 1643f - Trace Elements in Water
Material database: National Institute of Standards and Technology (NIST) - SRM 2976
Company database: PS Analytical
Material database: National Institute of Standards and Technology (NIST) - SRM 1946 Lake Superior Fish Tissue
Material database: National Research Council of Canada (NRC - CNRC) - DORM-4: Fish protein certified reference material for trace metals
Material database: National Institute of Standards and Technology (NIST) - SRM 1974b - Organics in Mussel Tissue (Mytilus edulis)
Link database: Cadmium and thyroid function
Material database: Sigma-Aldrich - Matrix CRM - Soil, Sludge, Sediment: TCLP Metals - Loamy Sand 3
Material database: National Institute of Standards and Technology (NIST) - SRM 1633b - Constituent Elements in Coal Fly Ash
Material database: National Institute for Environmental Studies (NIES) - CRM 13: Human Hair
Link database: US EPA: Basic Information about Selenium in Drinking Water
Material database: IRMM - Institute for Reference Materials and Measurements - BCR-637