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:

Directory of scientists: Vekoslava Stibilj
Material database: National Institute of Standards and Technology (NIST) - SRM 363
Selenium species are partly converted during their enzymatic extraction from biological samples with protease XIV | EVISA's News
Material database: National Metrology Institute of Japan (NMIJ) - NMIJ CRM 7402-a Trace Elements, Arsenobetaine and Methylmercury in Cod Fish Tissue
Link database: Australian Government: National Pollutant Inventory: Selenium & compounds
Material database: International Atomic Energy Agency (IAEA) - IAEA-155 Trace Elements in Whey Powder
Link database: Cadmium and thyroid function
Material database: National Institute of Standards and Technology (NIST) - SRM 3149
Elemental speciation is going commercial | EVISA's News
Material database: IAEA - MEL International Atomic Energy Agency - IAEA-405 - Trace Elements and Methylmercury in Estuarine Sediment
Material database: IRMM - Institute for Reference Materials and Measurements - BCR-063R Skim milk powder (natural)
Company database: Aqua Metrology Systems, Ltd.
Material database: National Institute of Standards and Technology (NIST) - SRM 1643e - Trace Elements in Water
Material database: RECIPE Chemicals + Instruments GmbH - ClinChek 8883 Control blood plasma, lyophilised, trace elements level I
New selenium metabolites found in human serum | EVISA's News
Material database: Sigma-Aldrich - Matrix CRM - Soil, Sludge, Sediment: Trace Metals - Fresh Water Sediment 3
Material database: JRC - Joint Research Centre - ERM-CD200: Bladderwrack (Fucus vesiculosus):
Application of Mass Spectrometry to Speciation Analysis in the Life Sciences | EVISA's Agenda of Events
Selenoprotein P is required for normal sperm development | EVISA's News
Material database: National Institute of Standards and Technology (NIST) - SRM 1849a - Infant/Adult Nutritional Formula