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: Food Analysis Performance Assessment Scheme (FAPAS) - Test Material T1094 (Dairy Ration)
Material database: IRMM - Institute for Reference Materials and Measurements - BCR-397
Instrument database: Analytik Jena GmbH - contrAAź 800 G - Graphite Furnace Atomic Absorption Spectrometer
Application of Mass Spectrometry to Speciation Analysis in the Life Sciences | EVISA's Agenda of Events
Material database: International Atomic Energy Agency (IAEA) - IAEA-336 Trace and Minor Elements in Lichen
Material database: IRMM - Institute for Reference Materials and Measurements - BCR-664
Link database: Hazardous Substances Data Bank: Selenium dimethyldithiocarbamate
Gas chromatography for the separation of elemental species | EVISA's News
Directory of scientists: Oto Mestek
Link database: Selenium and its Relationship to Cancer
Selenium species are partly converted during their enzymatic extraction from biological samples with protease XIV | EVISA's News
Material database: Sigma-Aldrich - Matrix CRM - Soil, Sludge, Sediment: TCLP Metals - Sandy Loam 2
Material database: JRC - Joint Research Centre - BCR-032 - Trace elements in Moroccan phosphate rock
Material database: RECIPE Chemicals + Instruments GmbH - ClinChek 8840 Whole blood, lyophilized, trace elements level I: LOT 445
Material database: National Institute for Minamata Disease (NIMD) - NIMD-1 Human Hair
Material database: National Institute of Standards and Technology (NIST) - SRM 3149 - Selenium Standard Solution
Improving the Determination of Toxic Metal Species in Drugs | EVISA's News
Link database: CODA-CERVA: Selenium
4th International Conference on Selenium in the Environment and Human Health | EVISA's Agenda of Events
Material database: National Research Council of Canada (NRC - CNRC) - PACS-2 - Marine Sediment Reference Materials for Trace Metals and other Constituents