https://en.wikipedia.org/wiki/Selenium
Selenium is a
chemical element with symbol
Se and
atomic number 34. It is a
nonmetal with properties that are intermediate between the elements above and below in the
periodic table,
sulfur and
tellurium. It rarely occurs in its elemental state or as pure ore compounds in the Earth's crust. Selenium (
Greek σελήνη
selene meaning "Moon") was discovered in 1817 by
Jöns Jacob Berzelius, who noted the similarity of the new element to the previously discovered
tellurium (named for the Earth).
Selenium is found in
metal sulfide ores, where it partially replaces the sulfur. Commercially, selenium is produced as a byproduct in the refining of these ores, most often during production. Minerals that are pure selenide or selenate compounds are known but rare. The chief commercial uses for selenium today are
glassmaking and
pigments. Selenium is a
semiconductor and is used in
photocells. Applications in
electronics, once important, have been mostly supplanted by
silicon semiconductor devices. Selenium is still used in a few types of DC power
surge protectors and one type of
fluorescent quantum dot.
Selenium salts are toxic in large amounts, but trace amounts are necessary for cellular function in many organisms, including all animals. Selenium is an ingredient in many multivitamins and other dietary supplements, including infant formula. It is a component of the antioxidant 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 requiring relatively large amounts and others apparently requiring none.[5]