|Name, Symbol, Number||selenium, Se, 34|
|Group, Period, Block||16, 4, p|
|Appearance||gray-black, metallic luster
|Atomic mass||78.96(3) g/mol|
|Electron configuration||[Ar] 3d10 4s2 4p4|
|Electrons per shell||2, 8, 18, 6|
|Density (near r.t.)||(gray) 4.81 g/cm³|
|Density (near r.t.)||(alpha) 4.39 g/cm³|
|Density (near r.t.)||(vitreous) 4.28 g/cm³|
|Liquid density at m.p.||3.99 g/cm³|
|Melting point||494 K
(221 °C, 430 °F)
|Boiling point||958 K
(685 °C, 1265 °F)
|Critical point||1766 K, 27.2 MPa|
|Heat of fusion||(gray) 6.69 kJ/mol|
|Heat of vaporization||95.48 kJ/mol|
|Heat capacity||(25 °C) 25.363 J/(mol·K)|
|Oxidation states||±2, 4, 6
(strongly acidic oxide)
|Electronegativity||2.55 (Pauling scale)|
|1st: 941.0 kJ/mol|
|2nd: 2045 kJ/mol|
|3rd: 2973.7 kJ/mol|
|Atomic radius||115 pm|
|Atomic radius (calc.)||103 pm|
|Covalent radius||116 pm|
|Van der Waals radius||190 pm|
|Magnetic ordering||no data|
|Thermal conductivity||(300 K) (amorphous)
|Thermal expansion||(25 °C) (amorphous)
|Speed of sound (thin rod)||(20 °C) 3350 m/s|
|Speed of sound (thin rod)||(r.t.) 10 m/s|
|Shear modulus||3.7 GPa|
|Bulk modulus||8.3 GPa|
|Brinell hardness||736 MPa|
|CAS registry number||7782-49-2|
Selenium (chemical symbol Se, atomic number 34) is a chemical element that is classified as a nonmetal. It is chemically related to sulfur and tellurium. Although toxic in excessive quantities, it is essential in trace amounts for the function of certain enzymes in humans and perhaps all other living organisms. It is therefore often added (in the form of sodium selenite) to vitamins and food supplements.
This element and its compounds have a wide range of applications. For example, selenium (or selenium dioxide) is used to impart a red color to glasses and enamels, and to remove the blue-green tinge in glass caused by ferrous impurities. It is an important catalyst for chemical reactions in industry as well as laboratory research. Combined with bismuth, it is added to brasses to replace lead. It is used in toners for photocopiers. Also, when used in the toning of photographic prints, it increases the permanence of the images. Selenium sulfide is an antifungal agent added to shampoos for the treatment of dandruff. Zinc selenide is used in light-emitting diodes, diode lasers, and X-ray and gamma-ray detectors.
In nature, selenium is not found in the free state, but it occurs naturally in a number of inorganic forms such as selenide, selenate, and selenite. In soils, selenium is found most often in soluble forms such as selenate (analogous to sulfate), which are easily leached into rivers by runoff.
In living organisms, selenium is part of organic compounds such as dimethyl selenide, selenomethionine, and selenocysteine. In these compounds, selenium plays a role analogous to sulfur.
Natural sources of selenium include certain selenium-rich soils, and selenium that has been bioconcentrated by certain toxic plants such as locoweed. Anthropogenic sources of selenium include coal burning and the mining and smelting of sulfide ores .
Selenium is most commonly produced from selenides that are present in many sulfide ores, particularly those of copper, silver, and lead. It is obtained as a byproduct of processing of these ores, as it is extracted from the anode mud of copper refineries and the mud from the lead chambers of sulfuric acid plants. .  . These muds can be processed by various means to obtain free selenium.
Commonly, production begins by oxidation with sodium carbonate to produce sodium selenite. The sodium selenite is then acidified with sulfuric acid (H2SO4) to produce selenous acid (H2SeO3). The selenous acid is finally bubbled with sulfur dioxide to produce red, amorphous selenium.
Growth in selenium consumption was historically driven by steady development of new uses, including applications in rubber compounding, steel alloying, and selenium rectifiers. By 1970, selenium in rectifiers had largely been replaced by silicon, but its use as a photoconductor in plain-paper copiers had become its leading application. During the 1980s, the photoconductor application declined (although it was still a large end-use), as increasing numbers of copiers using organic photoconductors were produced. Currently, the largest use of selenium worldwide is in glass manufacturing, followed by uses in chemicals and pigments. Electronic use, despite a number of applications, is declining. 
In 1996, research showed a positive correlation between selenium supplementation and cancer prevention in humans. Nonetheless, widespread direct application of this important finding did not add significantly to demand, owing to the small doses required. In the late 1990s, the use of selenium (usually with bismuth) as an additive to plumbing brasses to meet no-lead environmental standards, became important. At present, total global production of selenium continues to increase modestly.
In the periodic table, selenium is located in group 16 (formerly group 6A), between sulfur and tellurium. It is thus a member of the oxygen family of elements, also called the chalcogens. In addition, it lies between arsenic and bromine in period 4.
When selenium is produced through chemical reactions, it invariably appears as the amorphous, reddish form—an insoluble, brick-red powder. When this form is rapidly melted, it converts to the black, vitreous (glasslike) form that is usually sold industrially as beads. The most thermodynamically stable and dense form of selenium is the electrically conductive gray (trigonal) form, composed of long, helical chains of selenium atoms. The conductivity of this form is notably light sensitive—it conducts electricity better in the light than in the dark, and it is used in photocells. Selenium also exists in three different, deep red, crystalline monoclinic forms, which are composed of eight-membered ring molecules (Se8), similar to many allotropes of sulfur. 
Selenium can combine with metals and oxygen to form selenides (such as sodium selenide, Na2Se), selenates (such as calcium selenate, CaSeO4), and selenites (such as sodium selenite, Na2SeO3). In this regard, it is analogous to sulfur, which forms sulfides, sulfates, and sulfites. Selenium also forms hydrogen selenide (H2Se), a colorless, flammable gas that is the most toxic compound of selenium.
Selenium has at least 29 isotopes. Five of these are stable, and six are nuclear isomers (see table on the right).
Although toxic in large doses, selenium is an essential micronutrient in all known forms of life. It is a component of the unusual amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient that functions as a cofactor for reduction by antioxidant enzymes such as glutathione peroxidases and thioredoxin reductase. It also plays a role in the functioning of the thyroid gland by participating as a cofactor for thyroid hormone deiodinases . Dietary selenium comes from cereals, meat, fish, and eggs. Liver and Brazil nuts are particularly rich sources of selenium. A list of selenium rich foods can be found at The Office of Dietary Supplements Selenium Fact Sheet.
Selenium is widely used in vitamins and food supplements, in small doses—typically 50–200 micrograms per day for adult humans. The recommended dietary allowance for adults is 55 micrograms per day. Some livestock feeds are fortified as well.
Selenium is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis  Symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage. Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary edema, and death..
Elemental selenium and most metallic selenides have relatively low toxicities, due to their low bioavailability. By contrast, selenates and selenites are very toxic, with modes of action similar to that of arsenic. Hydrogen selenide is an extremely toxic, corrosive gas. Organic compounds such as dimethyl selenide, selenomethionine, and selenocysteine have high bioavailability and are toxic in large doses.
Selenium poisoning of water systems may result from new agricultural runoff through normally dry lands. This process leaches natural soluble selenium into the water, which may then be concentrated in new "wetlands" as it evaporates. High selenium levels produced in this fashion have caused certain birth defects in wetland birds. 
Selenium deficiency is relatively rare in healthy, well-nourished individuals. It can occur in patients with severely compromised intestinal function, or those undergoing total parenteral nutrition. People dependent on food grown from selenium-deficient soil are also at risk.
Deficiency in selenium can lead to Keshan disease, which is potentially fatal. It also contributes (along with iodine deficiency) to Kashin-Beck disease . The primary symptom of Keshan disease is myocardial necrosis, which weakens the heart. Keshan disease also makes the body more susceptible to illness caused by other nutritional, biochemical, or infectious diseases. Kashin-Beck disease results in atrophy, degeneration*, and necrosis of cartilage tissue . These diseases are most common in certain parts of China, where the soil is extremely deficient in selenium. Studies in Jiangsu Province of China have indicated a reduction in the prevalence of these diseases by taking selenium supplements. Selenium deficiency has also been associated with goiter, cretinism, and recurrent miscarriage in humans.
Coloration of glass and ceramics (currently the main use, worldwide)
Manufacturing and materials
These uses, however, have been or are being replaced by silicon-based devices.
All links retrieved November 2, 2019.
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