Selenium

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.
 * Selenium** 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.

The most stable allotrope of selenium is a dense reddish-gray solid. In terms of structure, it adopts a helical polymeric chain. The Se-Se distance is 2.37 Å and the Se-Se-Se angle is 103°. It is a semiconductor with the unusual property of conducting electricity better in the light than in the dark, and is used in photocells. Gray selenium resists oxidation by air and is not attacked by non-oxidizing acids. With strong reducing agents, it forms polyselenides.
 * Physical Characteristics**

__** Uses for Selenium **__ Source: []
 * For __electronics__ industry.
 * As a nutritional supplement.
 * In glass industry.
 * As component of pigments in plastics, paints, enamels, inks, and rubber.
 * In the preparation of pharmaceuticals.
 * As a nutritional feed additive for poultry and livestock.
 * In pesticide formulations.
 * As an ingredient in anti-dandruff shampoos.
 * As a constituent of fungicides.
 * For toning of photographs.
 * Radioactive selenium is used in diagnostic medicine.

Native selenium is a rare mineral, which does not usually form good crystals, but, when it does, they are steep rhombohedrons or tiny acicular (hair-like) crystals. Isolation of selenium is often complicated by the presence of other compounds and elements. Selenium occurs naturally in a number of inorganic forms, including selenide-, selenate-, and selenite-containing minerals. In living systems, selenium is found in the amino acids selenomethionine, selenocysteine, and methylselenocysteine. In these compounds, selenium plays a role analogous to that of sulfur. Another naturally occurring organoselenium compound is dimethyl selenide. Certain solids are selenium-rich, and selenium can be bioconcentrated by certain plants. In soils, selenium most often occurs in soluble forms such as selenate (analogous to sulfate), which are leached into rivers very easily by runoff. Anthropogenic sources of selenium include coal burning and the mining and smelting of sulfide ores.
 * Occurrences**

Discovery of Selenium
Author: Dr. Doug Stewart Selenium lies beneath sulfur in Group 16 of the periodic table. The chemical behavior and reactions of these elements are similar. It is possible selenium was first observed in about the year 1300 by the alchemist Arnold of Villanova. Villanova lived from about 1235 to about 1310 and was trained in medicine at the Sorbonne in Paris, becoming physician to Pope Clement V. In the book //Rosarium Philosophorum// he describes red sulfur or 'sulfur rebeum' which had been left behind in an oven after native sulfur had been vaporized. This may have been one of selenium's red colored allotropes. (1), (2), (3) There is no more to be said about selenium's discovery until 500 years later, in 1817. In this year, the eminent Swedish chemist Jöns J. Berzelius had his attention drawn to a red deposit left behind after sulfur was burned in a sulfuric acid factory. (4) The factory was actually part owned by Berzelius with his friend the chemist Johann Gahn. (5) Writing about the deposit in September 1817, Berzelius informed his friend in London, Dr. Marcet, that the deposit contained the (already known) element tellurium. In February 1818, however, he let Marcet know he had changed his mind, and told him of his discovery of a new element: "...what Mr. Gahn and I took for tellurium is a new substance, endowed with interesting properties. This substance has the properties of a metal, combined with that of sulfur to such a degree that one would say it is a new kind of sulfur. The similarity to tellurium has given me occasion to name the new substance selenium." (6) To explain Berzelius's name for the new element a little more: 'Tellus' in Latin means earth goddess. Berzelius took selenium from the Greek word 'Selene', meaning moon goddess. (Tellurium had been given its name in 1799 by the German chemist Martin Klaporth, who wrote, "No single element was yet named after the Earth. It needed to be done!)

Selenium is most commonly produced from selenide in many sulfide ores, such as those of copper, silver, or lead. It is obtained as a byproduct of the processing of these ores, e.g., from the anode mud of copper refineries and the mud from the lead chambers of sulfuric acid plants. These muds can be processed by a number of means to obtain selenium. Specifically, most elemental selenium comes as a byproduct of refining copper or producing sulfuric acid. Industrial production of selenium often involves the extraction of selenium dioxide from residues obtained during the purification of copper. Common production begins by oxidation with sodium carbonate to produce selenium dioxide. The selenium dioxide is then mixed with water and the solution is acidified to form selenous acid ( oxidation __step__). Selenous acid is bubbled with sulfur dioxide (reduction step) to give elemental selenium.
 * Production of Selenium**


 * Main Use for Selenium**

Glass production
The largest commercial use of Se, accounting for about 50% of consumption, is for the production of glass. Se compounds confer a red color to glass. This color cancels out the green or yellow tints that arise from iron impurities that are typical for most glass. For this purpose various selenite and selenate salts are added. For other applications, the red color may be desirable, in which case mixtures of CdSe and CdS are added.

=Health Benefits= Selenium is an essential micronutrient for animals In plants, it occurs as a bystander mineral Selenium is a component of the unusual amino acids selenocysteine and selenomethionine In humans, selenium is a trace element nutrient that functions as co-factor for reduction of antioxidant enzymes, such as glutathione peroxidases and certain forms of thioredoxin reductase found in animals and some plants Selenium also plays a role in the functioning of the thyroid gland and in every __cell__ that uses thyroid hormone, by participating as a co-factor for the three known thyroid hormone deiodinases, which activate and then deactivate various thyroid hormones and their metabolites. Toxic if taken in excess. The glutathione peroxidase family of enzymes (GSH-Px) catalyze certain reactions that remove reactive oxygen species such as hydrogen peroxide and organic hydroperoxides : 2 GSH + H 2 O 2 GSH-Px → GSSG + 2 H 2 O

**Having a Selenium Deficiency**
Human selenium deficiency is rare in the U.S. but is seen in other countries, most notably China, where soil concentration of selenium is low; there is evidence that selenium deficiency may contribute to development of a form of heart disease, hypothyroidism, and a weakened immune system. There is also evidence that selenium deficiency does not usually cause illness by itself. Rather, it can make the body more susceptible to illnesses caused by other nutritional, biochemical or infectious stresses. You can get supplements of selenium; Selenomethionine can be incorporated into body proteins in place of methionine, and serves as a vehicle for selenium storage in organs and tissues. Selenium supplements may also contain sodium selenite and sodium selenate, two inorganic forms of selenium. Selenomethionine is generally considered to be the best absorbed and utilized form of selenium.