Selenium (/sɨˈliːniəm/ sə-lee-nee-əm) is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium. It rarely occurs in its elemental state in nature, but instead is obtained as a side-product in the refining of other elements.

Selenium is found in sulfide ores such as pyrite, where it partially replaces the sulfur. Minerals that are selenide or selenate compounds are also known, but are rare. The chief commercial uses for selenium today are in glassmaking and in pigments. Uses in electronics, once important, have been supplanted by silicon semiconductor devices. It is a semiconductor with the unusual property of conducting electricity better in the light than in the dark, and is used in photocells. Selenium is a trace mineral that is essential to good health but required only in small amounts. Selenium is incorporated into proteins to make selenoproteins, which are important antioxidant enzymes. The antioxidant properties of selenoproteins help prevent cellular damage from free radicals. Free radicals are natural by-products of oxygen metabolism that may contribute to the development of chronic diseases such as cancer and heart disease. Other selenoproteins help regulate thyroid function and play a role in the immune system.

Selenium salts are toxic in large amounts, but trace amounts are necessary for cellular function in many organisms. It is a component of the enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants). It is also found in three deiodinase enzymes, which convert one thyroid hormone to another. Selenium requirements in plants differ by species, with some plants, it seems, requiring none.

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