Selenium
Overview
 
Selenium is a chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 with atomic number
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...

 34, chemical symbol
Chemical symbol
A chemical symbol is a 1- or 2-letter internationally agreed code for a chemical element, usually derived from the name of the element, often in Latin. Only the first letter is capitalised...

 Se, and an atomic mass
Atomic mass
The atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....

 of 78.96. It is a nonmetal
Nonmetal
Nonmetal, or non-metal, is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a nonmetal...

, whose properties are intermediate between those of adjacent chalcogen
Chalcogen
The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family...

 elements sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 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
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

 ores such as pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...

, where it partially replaces the sulfur.
Encyclopedia
Selenium is a chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 with atomic number
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...

 34, chemical symbol
Chemical symbol
A chemical symbol is a 1- or 2-letter internationally agreed code for a chemical element, usually derived from the name of the element, often in Latin. Only the first letter is capitalised...

 Se, and an atomic mass
Atomic mass
The atomic mass is the mass of a specific isotope, most often expressed in unified atomic mass units. The atomic mass is the total mass of protons, neutrons and electrons in a single atom....

 of 78.96. It is a nonmetal
Nonmetal
Nonmetal, or non-metal, is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a nonmetal...

, whose properties are intermediate between those of adjacent chalcogen
Chalcogen
The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family...

 elements sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 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
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

 ores such as pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...

, 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 pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

s. Uses in electronics
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

, once important, have been supplanted by silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 semiconductor devices. It is a semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 with the unusual property of conducting electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 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
Glutathione peroxidase
Glutathione peroxidase is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage...

 and thioredoxin reductase
Thioredoxin reductase
Thioredoxin Reductases are the only known enzymes to reduce thioredoxin . Two classes of thioredoxin reductase have been identified: one class in bacteria and some eukaryotes and one in animals. Both classes are flavoproteins which function as homodimers...

 (which indirectly reduce certain oxidized molecules in animals and some plants). It is also found in three deiodinase
Deiodinase
Iodothyronine deiodinases are a subfamily of deiodinase enzymes important in the activation and deactivation of thyroid hormones. Thyroxine , the precursor of 3,5,3’-triiodothyronine is transformed into T3 by deiodinase activity. T3, through binding a nuclear thyroid hormone receptor,...

 enzymes, which convert one thyroid hormone
Thyroid hormone
The thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine ,...

 to another. Selenium requirements in plants differ by species, with some plants, it seems, requiring none.

Physical

The most stable allotrope of selenium is a dense purplish-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
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 with the unusual property of conducting electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 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.

Red Se and related molecular allotropes

The second major allotrope of Se is red selenium. The solid consists of individual molecules of eight-membered ring
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...

 molecules, like its lighter cousin sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

. The Se-Se distance is 2.34 Å and the Se-Se-Se angle is 106°. Unlike sulfur, however, the red form converts to the gray polymeric allotrope with heat. Other rings with the formula Sen are also known.

Elemental selenium produced in chemical reactions invariably appears as the amorphous red form: an insoluble, brick-red powder. When this form is rapidly melted, it forms the black, vitreous form, which is usually sold industrially as beads. The red allotrope crystallises in three habits. Selenium does not exhibit the unusual changes in viscosity that sulfur undergoes when gradually heated.

Isotopes

Selenium has six naturally occurring isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

s, five of which are stable: 74Se, 76Se, 77Se, 78Se, and 80Se. The last three also occur as fission products, along with 79Se
Selenium-79
Selenium-79 is a radioisotope of selenium present in spent nuclear fuel and the wastes resulting from reprocessing this fuel. It is one of only 7 long-lived fission products. Its yield is low as it is near the lower end of the mass range for fission products...

, which has a half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...

 of 327,000 years. The final naturally occurring isotope, 82Se, has a very long half-life (~1020 yr, decaying via double beta decay to 82Kr
Krypton
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...

), which, for practical purposes, can be considered to be stable. Twenty-three other unstable isotopes have been characterized.

See also Selenium-79
Selenium-79
Selenium-79 is a radioisotope of selenium present in spent nuclear fuel and the wastes resulting from reprocessing this fuel. It is one of only 7 long-lived fission products. Its yield is low as it is near the lower end of the mass range for fission products...

for more information on recent changes in the measured half-life of this long-lived fission product, important for the dose calculations performed in the frame of the geological disposal of long-lived radioactive waste
Radioactive waste
Radioactive wastes are wastes that contain radioactive material. Radioactive wastes are usually by-products of nuclear power generation and other applications of nuclear fission or nuclear technology, such as research and medicine...

.

Occurrence


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
Selenide
A selenide is a chemical compound in which selenium serves as an anion with oxidation number of −2 , much as sulfur does in a sulfide. The chemistry of the selenides and sulfides are similar....

-, selenate
Selenate
The selenate ion is SeO42–.Selenates are analogous to sulfates and have similar chemistry. They are highly soluble in aqueous solutions at ambient temperatures....

-, and selenite
Selenite (ion)
The selenite anion is a selenium oxoanion with the chemical formula SeO32−.A selenite is a compound that contains this ion....

-containing minerals.

In living systems, selenium is found in the amino acids selenomethionine
Selenomethionine
Selenomethionine is an amino acid containing selenium. The L-enantiomer of selenomethionine, known as Se-met and Sem, is a common natural food source of selenium. In vivo, selenomethionine is randomly incorporated instead of methionine and is readily oxidized. Its antioxidant activity arises from...

, selenocysteine
Selenocysteine
Selenocysteine is an amino acid that is present in several enzymes .-Nomenclature:...

, and methylselenocysteine. In these compounds, selenium plays a role analogous to that of sulfur. Another naturally occurring organoselenium compounds is dimethyl selenide.

Certain solids are selenium-rich, and selenium can be bioconcentrated
Bioaccumulation
Bioaccumulation refers to the accumulation of substances, such as pesticides, or other organic chemicals in an organism. Bioaccumulation occurs when an organism absorbs a toxic substance at a rate greater than that at which the substance is lost...

 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.

Production

Selenium is most commonly produced from selenide
Selenide
A selenide is a chemical compound in which selenium serves as an anion with oxidation number of −2 , much as sulfur does in a sulfide. The chemistry of the selenides and sulfides are similar....

 in many sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...

 ores
Orés
Orés is a municipality in the Cinco Villas, in the province of Zaragoza, in the autonomous community of Aragon, Spain. It belongs to the comarca of Cinco Villas. It is placed 104 km to the northwest of the provincial capital city, Zaragoza. Its coordinates are: 42° 17' N, 1° 00' W, and is...

, such as those of copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

, or lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

. It is obtained as a byproduct of the processing of these ores, e.g., from the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 mud of copper refineries and the mud from the lead chambers
Lead chamber process
The lead chamber process was an industrial method used to produce sulfuric acid in large quantities. It has been largely supplanted by the contact process....

 of sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 plants. These muds can be processed by a number of means to obtain selenium. Specifically, most elemental selenium comes as a byproduct of refining
Refinery
A refinery is a production facility composed of a group of chemical engineering unit processes and unit operations refining certain materials or converting raw material into products of value.-Types of refineries:Different types of refineries are as follows:...

 copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 or producing sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

.

Industrial production of selenium often involves the extraction of selenium dioxide
Selenium dioxide
Selenium dioxide is the chemical compound with the formula SeO2. This colorless solid is one of the most frequently encountered compounds of selenium.-Properties:...

 from residues obtained during the purification of copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

. Common production begins by oxidation with sodium carbonate
Sodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...

 to produce selenium dioxide. The selenium dioxide is then mixed with water and the solution is acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ified to form selenous acid
Selenous acid
Selenous acid is the chemical compound with the formula H2SeO3. Structurally, it is more accurately described by 2SeO. It is the principal oxoacid of selenium; the other being selenic acid.-Formation and properties:...

 (oxidation step). Selenous acid is bubbled with sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

 (reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 step) to give elemental selenium.

Chemical compounds

Selenium compounds commonly exist in oxidation states -II, +II, +IV, and +VI.

Chalcogen compounds

Selenium forms two stable oxides: selenium dioxide
Selenium dioxide
Selenium dioxide is the chemical compound with the formula SeO2. This colorless solid is one of the most frequently encountered compounds of selenium.-Properties:...

  and selenium trioxide
Selenium trioxide
Selenium trioxide is a chemical compound used in the manufacture of photoelectric cells and solar energy devices.A laboratory preparation is to react anhydrous K2SeO4 and SO3. The compound is thermodynamically unstable with respect to selenium dioxide. The solid consists of cyclic tetramers,...

 . Selenium dioxide is formed by the reaction of elemental selenium with oxygen:
+ 8 → 8


It is a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

ic solid that forms monomeric molecules in the gas phase. It dissolves in water to form selenous acid
Selenous acid
Selenous acid is the chemical compound with the formula H2SeO3. Structurally, it is more accurately described by 2SeO. It is the principal oxoacid of selenium; the other being selenic acid.-Formation and properties:...

, . Selenous acid can also be made directly by oxidising elemental selenium with nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

:
3 Se + 4 → 3 + 4 NO

Unlike sulfur, which forms a stable trioxide
Sulfur trioxide
Sulfur trioxide is the chemical compound with the formula SO3. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. It is prepared on massive scales as a precursor to sulfuric acid.-Structure and bonding:Gaseous SO3 is a trigonal planar molecule of...

, selenium trioxide is unstable and decomposes to the dioxide above 185 °C:
2 → 2 + (ΔH = −54 kJ/mol)


Salts of selenous acid are called selenites. These include silver selenite
Silver selenite
Silver selenite is an inorganic compound of formula Ag2SeO3.-Production:It is formed during recovery from copper anode slimes when they are subjected to oxidative roasting where a part of silver selenide is converted to the selenite.-External links:*...

  and sodium selenite
Sodium selenite
Sodium selenite is a salt, a colourless solid, and the most common water-soluble selenium compound. It has the formulas Na2SeO3 and Na2SeO35 . Respectively, these are the anhydrous salt and its pentahydrate. This hydrated salt is the more common one...

 .

Hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

 reacts with aqueous selenous acid to produce selenium disulfide:
+ 2 → + 3


Selenium disulfide consists of 8-membered rings of a nearly statistical distribution of sulfur and selenium atoms. It has an approximate composition of , with individual rings varying in composition, such as and . Selenium disulfide has been use in shampoo as an anti-dandruff
Dandruff
Dandruff is the shedding of dead skin cells from the scalp . Dandruff is sometimes caused by frequent exposure to extreme heat and cold. As it is normal for skin cells to die and flake off, a small amount of flaking is normal and common; about 487,000 cells/cm2 get released normally after...

 agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in fireworks
Fireworks
Fireworks are a class of explosive pyrotechnic devices used for aesthetic and entertainment purposes. The most common use of a firework is as part of a fireworks display. A fireworks event is a display of the effects produced by firework devices...

.

Selenium trioxide may be synthesized by dehydrating selenic acid
Selenic acid
Selenic acid is the chemical compound with the formula . It is an oxoacid of selenium, and its structure is more accurately described as .As predicted by VSEPR theory, the selenium center is tetrahedral, with a Se–O bond length of 161 pm...

, , which is itself produced by the oxidation of selenium dioxide with hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

:
+ →


Hot, concentrated selenic acid is capable of dissolving gold, forming gold(III) selenate.

Halogen compounds

Iodides of selenium are not well known. The only stable chloride is Se2Cl2
Selenium monochloride
Selenium monochloride is an inorganic compound with the formula Se2Cl2. Although it is called selenium monochloride, a more descriptive name might be diselenium dichloride. It is a reddish-brown, oily liquid that is hydrolyses slowly. It exists in chemical equilibrium with SeCl2, SeCl4, chlorine,...

; the corresponding bromide is also known. These species are structurally analogous to the corresponding disulfur dichloride
Disulfur dichloride
Disulfur dichloride is the chemical compound with the formula S2Cl2 . Some alternative names for this compound are sulfur monochloride , disulphur dichloride and sulphur monochloride...

. Selenium dichloride is an important reagent in the preparation of selenium compounds (e.g. the preparation of Se7). It is prepared by treating selenium with
Sulfuryl chloride
Sulfuryl chloride is an inorganic compound with the formula SO2Cl2. At room temperature, it is a colorless liquid with a pungent odor. Sulfuryl chloride is not found in nature, as can be inferred from its rapid hydrolysis....

. Selenium reacts with fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

 to form selenium hexafluoride
Selenium hexafluoride
Selenium hexafluoride is the inorganic compound with the formula SeF6. It is a colourless gas described as having a "repulsive" odor. It is not widely encountered and has no commercial applications.-Structure, preparation, and reactions:...

:
+ 24 → 8


In comparison with its sulfur counterpart (sulfur hexafluoride
Sulfur hexafluoride
Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...

), is more reactive and is toxic pulmonary irritant.
Some of the selenium oxyhalides, such as and selenium oxychloride have been used as specialty solvents.

Selenides

Analogous to the behavior of other chalcogens, selenium forms a dihydride . It is a strongly odiferous, toxic, and colourless gas. It is more acidic than . In solution it ionizes to HSe-. The selenide dianion Se2- forms a variety of compounds, including the minerals from which selenium is obtained commercially. Illustrative selenides include mercury selenide (HgSe), lead selenide (PbSe), zinc selenide
Zinc selenide
Zinc selenide , is a light yellow binary solid compound. It is an intrinsic semiconductor with a band gap of about 2.70 eV at 25 °C. ZnSe rarely occurs in nature...

 (ZnSe), and copper indium gallium diselenide . These materials are semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

s. With highly electropositive metals, such as aluminium, these selenides are prone to hydrolysis:
+ 6 H2O → 8 + H2Se

Alkali metal selenides react with selenium to form polyselenides, Sex2-, which exist as chains.

Other compounds

Tetraselenium tetranitride, , is an explosive orange compound analogous to
Tetrasulfur tetranitride
Tetrasulfur tetranitride is an inorganic compound with the formula S4N4. This gold-poppy coloured solid is the most important binary sulfur nitride, which are compounds that contain only the elements sulfur and nitrogen. It is a precursor to many S-N compounds and has attracted wide interest for...

. It can be synthesized by the reaction of with .

Selenium reacts with cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....

s to yield selenocyanates:
8 KCN + → 8 KSeCN

Organoselenium compounds

Selenium, especially in the II oxidation state, forms stable bonds to carbon. Typical compounds include selenols with the formula RSeH (e.g., benzeneselenol
Benzeneselenol
Benzeneselenol is the chemical compound with the formula C6H5SeH, often abbreviated PhSeH. This intensely malodorous liquid is a useful reagent in organic synthesis.-Synthesis and basic properties:...

), selenides with the formula RSeR (e.g., diphenylselenide), and diselenides, with the formula RSeSeR (e.g., diphenyldiselenide). Selenoxides, with the formula RSe(O)R, and selenyl chlorides, with the formula RSeCl, are useful intermediates in organic chemistry.

History and global demand

Selenium (Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 σελήνη selene
Selene
In Greek mythology, Selene was an archaic lunar deity and the daughter of the Titans Hyperion and Theia. In Roman mythology, the moon goddess is called Luna, Latin for "moon"....

meaning "Moon") was discovered in 1817 by Jöns Jakob Berzelius
Jöns Jakob Berzelius
Jöns Jacob Berzelius was a Swedish chemist. He worked out the modern technique of chemical formula notation, and is together with John Dalton, Antoine Lavoisier, and Robert Boyle considered a father of modern chemistry...

, who found the element associated with tellurium (named for the Earth). It was discovered as a byproduct of sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 production.

It came to medical notice later because of its toxicity to humans working in industry. It was also recognized as an important veterinary toxin, seen in animals eating high-selenium plants. In 1954, the first hints of specific biological functions of selenium were discovered in microorganisms. Its essentiality for mammalian life was discovered in 1957. In the 1970s, it was shown to be present in two independent sets of enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s. This was followed by the discovery of selenocysteine
Selenocysteine
Selenocysteine is an amino acid that is present in several enzymes .-Nomenclature:...

 in proteins. During the 1980s, it was shown that selenocysteine is encoded by the codon TGA
Genetic code
The genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells....

. The recoding mechanism was worked out first in bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and then in mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s (see SECIS element
SECIS element
In biology, the SECIS element is an RNA element around 60 nucleotides in length that adopts a stem-loop structure. This structural motif directs the cell to translate UGA codons as selenocysteines...

).

Health effects and nutrition

Although it is toxic in large doses, selenium is an essential micronutrient
Micronutrient
Micronutrients are nutrients required by humans and other living things throughout life in small quantities to orchestrate a whole range of physiological functions, but which the organism itself cannot produce. For people, they include dietary trace minerals in amounts generally less than 100...

 for animals. In plants, it occurs as a bystander mineral, sometimes in toxic proportions in forage
Forage
Forage is plant material eaten by grazing livestock.Historically the term forage has meant only plants eaten by the animals directly as pasture, crop residue, or immature cereal crops, but it is also used more loosely to include similar plants cut for fodder and carried to the animals, especially...

 (some plants may accumulate selenium as a defense against being eaten by animals, but other plants such as locoweed
Locoweed
Locoweed is a common name in North America for any plant that produces swainsonine, a phytotoxin harmful to livestock. Worldwide, swainsonine is produced by a small number of species, most in three genera of the flowering plant family Fabaceae: Oxytropis and Astragalus in North America, and...

 require selenium, and their growth indicates the presence of selenium in soil). See more on plant nutrition below.

Selenium is a component of the unusual amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s selenocysteine
Selenocysteine
Selenocysteine is an amino acid that is present in several enzymes .-Nomenclature:...

 and selenomethionine
Selenomethionine
Selenomethionine is an amino acid containing selenium. The L-enantiomer of selenomethionine, known as Se-met and Sem, is a common natural food source of selenium. In vivo, selenomethionine is randomly incorporated instead of methionine and is readily oxidized. Its antioxidant activity arises from...

. In humans, selenium is a trace element
Dietary mineral
Dietary minerals are the chemical elements required by living organisms, other than the four elements carbon, hydrogen, nitrogen, and oxygen present in common organic molecules. Examples of mineral elements include calcium, magnesium, potassium, sodium, zinc, and iodine...

 nutrient that functions as cofactor
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....

 for reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 of antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

 enzymes, such as glutathione peroxidase
Glutathione peroxidase
Glutathione peroxidase is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage...

s and certain forms of thioredoxin reductase
Thioredoxin reductase
Thioredoxin Reductases are the only known enzymes to reduce thioredoxin . Two classes of thioredoxin reductase have been identified: one class in bacteria and some eukaryotes and one in animals. Both classes are flavoproteins which function as homodimers...

 found in animals and some plants (this enzyme occurs in all living organisms, but not all forms of it in plants require selenium).

The glutathione peroxidase
Glutathione peroxidase
Glutathione peroxidase is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage...

 family of enzymes (GSH-Px) catalyze certain reactions that remove reactive oxygen species such as hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

 and organic hydroperoxides:
2 GSH + H2O2----GSH-Px → GSSG + 2 H2O


Selenium also plays a role in the functioning of the thyroid
Thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...

 gland and in every cell that uses thyroid hormone, by participating as a cofactor
Cofactor (biochemistry)
A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....

 for the three known thyroid hormone
Thyroid hormone
The thyroid hormones, thyroxine and triiodothyronine , are tyrosine-based hormones produced by the thyroid gland primarily responsible for regulation of metabolism. An important component in the synthesis of thyroid hormones is iodine. The major form of thyroid hormone in the blood is thyroxine ,...

 deiodinase
Deiodinase
Iodothyronine deiodinases are a subfamily of deiodinase enzymes important in the activation and deactivation of thyroid hormones. Thyroxine , the precursor of 3,5,3’-triiodothyronine is transformed into T3 by deiodinase activity. T3, through binding a nuclear thyroid hormone receptor,...

s, which activate and then deactivate various thyroid hormones and their metabolites. It may inhibit Hashimotos's disease
Hashimoto's thyroiditis
Hashimoto's thyroiditis or chronic lymphocytic thyroiditis is an autoimmune disease in which the thyroid gland is gradually destroyed by a variety of cell- and antibody-mediated immune processes. It was the first disease to be recognized as an autoimmune disease...

, in which the body's own thyroid cells are attacked as alien. A reduction of 21% on TPO antibodies was reported with the dietary intake of 0.2 mg of selenium.

Dietary sources of Se

Dietary selenium comes from nuts, cereals, meat, mushrooms, fish, and eggs. Brazil nut
Brazil Nut
The Brazil nut is a South American tree in the family Lecythidaceae, and also the name of the tree's commercially harvested edible seed.- Order :...

s are the richest ordinary dietary source (though this is soil-dependent, since the Brazil nut does not require high levels of the element for its own needs). In descending order of concentration, high levels are also found in kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

, tuna
Tuna
Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...

, crab
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

, and lobster
Lobster
Clawed lobsters comprise a family of large marine crustaceans. Highly prized as seafood, lobsters are economically important, and are often one of the most profitable commodities in coastal areas they populate.Though several groups of crustaceans are known as lobsters, the clawed lobsters are most...

.

The human body's content of selenium is believed to be in the 13–20 milligram range.

Indicator plants

Certain species of plants are considered indicators of high selenium content of the soil, since they require high levels of selenium to thrive. The main selenium indicator plants are Astragalus
Astragalus
Astragalus is a large genus of about 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae, subfamily Faboideae. The genus is native to temperate regions of the Northern Hemisphere...

species (including some locoweed
Locoweed
Locoweed is a common name in North America for any plant that produces swainsonine, a phytotoxin harmful to livestock. Worldwide, swainsonine is produced by a small number of species, most in three genera of the flowering plant family Fabaceae: Oxytropis and Astragalus in North America, and...

s), prince's plume (Stanleya
Stanleya (plant)
Stanleya is a genus of six species of plants in the mustard family known commonly as prince's plumes. These are herbs or erect shrubs which bear dense, plumelike inflorescences of white to bright yellow flowers with long stamens. Stanleya species are native to the western United States...

sp.), woody asters (Xylorhiza
Xylorhiza
Xylorhiza is a genus of flowering plants in the daisy family which are known as woodyasters. These are daisylike wildflowers usually having blue to purple or white ray flowers with yellow centers of disc florets...

sp.), and false goldenweed (Oonopsis
Oonopsis
Oonopsis is a genus of flowering plants in the daisy family, Asteraceae....

sp.)

Toxicity

Although selenium is an essential trace element
Dietary mineral
Dietary minerals are the chemical elements required by living organisms, other than the four elements carbon, hydrogen, nitrogen, and oxygen present in common organic molecules. Examples of mineral elements include calcium, magnesium, potassium, sodium, zinc, and iodine...

, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level
Dietary Reference Intake
The Dietary Reference Intake is a system of nutrition recommendations from the Institute of Medicine of the U.S. National Academy of Sciences. The DRI system is used by both the United States and Canada and is intended for the general public and health professionals...

 of 400 micrograms per day can lead to selenosis. This 400 microgram (µg
Microgram
In the metric system, a microgram is a unit of mass equal to one millionth of a gram , or 1/1000 of a milligram. It is one of the smallest units of mass commonly used...

) Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow up study on the same five people in 1992. The 1992 study actually found the maximum safe dietary Se intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to not only avoid toxicity
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

, but also to avoid creating an imbalance of nutrients in the diet and to account for data from other countries. In China, people who ingested corn grown in extremely selenium-rich stony coal (carbonaceous shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

) have suffered from selenium toxicity. This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded in literature.
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
Cirrhosis
Cirrhosis is a consequence of chronic liver disease characterized by replacement of liver tissue by fibrosis, scar tissue and regenerative nodules , leading to loss of liver function...

 of the liver, pulmonary edema
Pulmonary edema
Pulmonary edema , or oedema , is fluid accumulation in the air spaces and parenchyma of the lungs. It leads to impaired gas exchange and may cause respiratory failure...

, and death. Elemental selenium and most metallic selenide
Selenide
A selenide is a chemical compound in which selenium serves as an anion with oxidation number of −2 , much as sulfur does in a sulfide. The chemistry of the selenides and sulfides are similar....

s have relatively low toxicities because of their low bioavailability
Bioavailability
In pharmacology, bioavailability is a subcategory of absorption and is used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered...

. By contrast, selenate
Selenate
The selenate ion is SeO42–.Selenates are analogous to sulfates and have similar chemistry. They are highly soluble in aqueous solutions at ambient temperatures....

s and selenites
Selenite (ion)
The selenite anion is a selenium oxoanion with the chemical formula SeO32−.A selenite is a compound that contains this ion....

 are very toxic, having an oxidant mode of action similar to that of arsenic trioxide. The chronic toxic dose of selenite for humans is about 2400 to 3000 micrograms of selenium per day for a long time. Hydrogen selenide
Hydrogen selenide
Hydrogen selenide is the inorganic compound with the formula H2Se. It is the simplest and virtually the only hydride of selenium. H2Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound with an exposure limit: 0.05 ppm over an 8 hour period...

 is an extremely toxic, corrosive gas. Selenium also occurs in organic compounds, such as dimethyl selenide, selenomethionine
Selenomethionine
Selenomethionine is an amino acid containing selenium. The L-enantiomer of selenomethionine, known as Se-met and Sem, is a common natural food source of selenium. In vivo, selenomethionine is randomly incorporated instead of methionine and is readily oxidized. Its antioxidant activity arises from...

, selenocysteine
Selenocysteine
Selenocysteine is an amino acid that is present in several enzymes .-Nomenclature:...

 and methylselenocysteine, all of which have high bioavailability
Bioavailability
In pharmacology, bioavailability is a subcategory of absorption and is used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered...

 and are toxic in large doses.

On 19 April 2009, twenty-one polo
Polo
Polo is a team sport played on horseback in which the objective is to score goals against an opposing team. Sometimes called, "The Sport of Kings", it was highly popularized by the British. Players score by driving a small white plastic or wooden ball into the opposing team's goal using a...

 ponies began to die shortly before a match in the United States Polo Open. Three days later, a pharmacy released a statement explaining that the horses had received an incorrect dose of one of the ingredients used in a vitamin/mineral supplement compound, with which the horses had been injected. Such nutrient injections are common to promote recovery after a match, but this mixture had been compounded by a compounding pharmacy not familiar with it. Analysis of blood levels of inorganic compound
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

s in the supplement indicated the selenium concentrations were ten to fifteen times higher than normal in the horses' blood sample
Blood Sample
Blood Sample is the 10th studio album by the Finnish avant-garde progressive metal band Waltari.-Track listing:# "Helsinki" - 5:53# "Not Enough" - 3:35# "Too Much Emptiness" - 2:30# "Never" - 4:01# "New York" - 3:48# "I'm in Pain" - 4:38...

s, and 15 to 20 times higher than normal in their liver samples. It was later confirmed that selenium was the ingredient in question. Selenium is active in only tiny amounts, and has a history of causing accidental poisonings in supplements when the dose that is supposed to be in microgram
Microgram
In the metric system, a microgram is a unit of mass equal to one millionth of a gram , or 1/1000 of a milligram. It is one of the smallest units of mass commonly used...

s is given by mistake in milligrams (1000 times as much).

Selenium poisoning
Selenium pollution
In high concentrations, selenium acts as an environmental contaminate. Sources of pollution include waste materials from certain mining, agricultural, petrochemical, and industrial manufacturing operations. In Belews Lake North Carolina, 19 species of fish were eliminated from the lake due to...

 of water systems may result whenever new agricultural runoff courses through normally dry, undeveloped lands. This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in new "wetlands" as the water evaporates. High selenium levels produced in this fashion have been found to have caused certain congenital disorders in wetland birds.

Deficiency

Selenium deficiency is rare in healthy, well-nourished individuals. It can occur in patients with severely compromised intestinal
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...

 function, those undergoing total parenteral nutrition
Total parenteral nutrition
Parenteral nutrition is feeding a person intravenously, bypassing the usual process of eating and digestion. The person receives nutritional formulae that contain nutrients such as glucose, amino acids, lipids and added vitamins and dietary minerals...

, and in those of advanced age (over 90). Also, people dependent on food grown from selenium-deficient soil are at risk. Although New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

 has low levels of selenium in its soil, adverse health effects have not been detected.

Selenium deficiency may only occur when a low selenium status is linked with an additional stress, such as chemical exposure or increased oxidant stress due to vitamin E deficiency.

There are interactions between selenium and other nutrients, such as iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

 and vitamin E
Vitamin E
Vitamin E is used to refer to a group of fat-soluble compounds that include both tocopherols and tocotrienols. There are many different forms of vitamin E, of which γ-tocopherol is the most common in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine and dressings...

. The interaction is observed in the etiology
Etiology
Etiology is the study of causation, or origination. The word is derived from the Greek , aitiologia, "giving a reason for" ....

 of many deficiency diseases in animals and pure selenium deficiency is rare. The effect of selenium deficiency on health remains uncertain, particularly in relation to Kashin-Beck disease
Kashin-Beck Disease
Kashin-Beck disease is a chronic, endemic osteochondropathy , which is mainly distributed from northeastern to southwestern China involving 15 provinces . Tibet currently has the highest incidence rate of KBD in China . Southeast Siberia and North Korea are also other affected areas...

.

Controversial health effects

Cancer

Several studies have suggested a possible link between cancer and selenium deficiency. One study, known as the NPC, was conducted to test the effect of selenium supplementation on the recurrence of skin cancers on selenium-deficient men. It did not demonstrate a reduced rate of recurrence of skin cancers, but did show a reduced occurrence of total cancers, particularly for lung, colorectal and prostate cancers (Relative Risk 0.63). There was also a significant reduction in total cancer mortality (−50 %), although without a statistically significant change in overall mortality. The preventative effect observed in the NPC was greatest in those with the lowest baseline selenium levels. In 2009, the 5.5 year SELECT study reported selenium and vitamin E supplementation, both alone and together, did not significantly reduce the incidence of prostate cancer
Prostate cancer
Prostate cancer is a form of cancer that develops in the prostate, a gland in the male reproductive system. Most prostate cancers are slow growing; however, there are cases of aggressive prostate cancers. The cancer cells may metastasize from the prostate to other parts of the body, particularly...

 in 35,000 men who "generally were replete in selenium at baseline". The SELECT trial reported vitamin E did not reduce prostate cancer as it had in the alpha-tocopherol, beta carotene (ATBC) study, but the ATBC had a large percentage of smokers, while the SELECT trial did not. There was a slight trend toward more prostate cancer in the SELECT trial, but in the vitamin E only arm of the trial, where no selenium was given.

Dietary selenium prevents chemically induced carcinogenesis in many rodent studies. It has been proposed that selenium may help prevent cancer by acting as an antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

 or by enhancing immune activity. Not all studies agree on the cancer-fighting effects of selenium. One study of naturally occurring levels of selenium in over 60,000 participants did not show a significant correlation between those levels and cancer. The SU.VI.MAX study concluded low-dose supplementation (with 120 mg of ascorbic acid, 30 mg of vitamin E, 6 mg of beta carotene, 100 µg of selenium, and 20 mg of zinc) resulted in a 30% reduction in the incidence of cancer and a 37% reduction in all-cause mortality in males, but did not get a significant result for females. A Cochrane review of studies concluded that there is no convincing evidence that individuals, particularly those who are adequately nourished, will benefit from selenium supplementation with regard to their cancer risk. However, there is evidence selenium can help chemotherapy treatment by enhancing the efficacy of the treatment, reducing the toxicity of chemotherapeutic drugs, and preventing the body's resistance to the drugs. Studies of cancer cells in vitro showed that chemotherapeutic drugs, such as taxol and Adriamycin
Doxorubicin
Doxorubicin INN is a drug used in cancer chemotherapy. It is an anthracycline antibiotic, closely related to the natural product daunomycin, and like all anthracyclines, it works by intercalating DNA....

, were more toxic to strains of cancer cells when selenium was added.

In March 2009, vitamin E (400 IU) and selenium (200 micrograms) supplements were reported to affect gene expression and can act as a tumor suppressor. Eric Klein, MD from the Glickman Urological and Kidney Institute in Ohio said the new study “lend[s] credence to the previous evidence that selenium and vitamin E might be active as cancer preventatives”. In an attempt to rationalize the differences between epidemiological and in vitro studies and randomized trials like SELECT, Klein said randomized controlled trials “do not always validate what we believe biology indicates and that our model systems are imperfect measures of clinical outcomes in the real world”.

HIV/AIDS

Some research has indicated a geographical link between regions of selenium-deficient soils and peak incidences of HIV
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

/AIDS
AIDS
Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...

 infection. For example, much of sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa as a geographical term refers to the area of the African continent which lies south of the Sahara. A political definition of Sub-Saharan Africa, instead, covers all African countries which are fully or partially located south of the Sahara...

 is low in selenium. However, Senegal
Senegal
Senegal , officially the Republic of Senegal , is a country in western Africa. It owes its name to the Sénégal River that borders it to the east and north...

 is not, and also has a significantly lower level of AIDS infection than the rest of the continent. AIDS appears to involve a slow and progressive decline in levels of selenium in the body. Whether this decline in selenium levels is a direct result of the replication of HIV or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated.

Low selenium levels in AIDS patients have been directly correlated with decreased immune cell count and increased disease progression and risk of death. Selenium normally acts as an antioxidant
Antioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...

, so low levels of it may increase oxidative stress on the immune system, leading to its more rapid decline. Others have argued T-cell-associated genes encode selenoproteins similar to human glutathione peroxidase
Glutathione peroxidase
Glutathione peroxidase is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage...

. Depleted selenium levels in turn lead to a decline in CD4 helper T-cells, further weakening the immune system.

Regardless of the cause of depleted selenium levels in AIDS patients, studies have shown selenium deficiency does strongly correlate with the progression of the disease and the risk of death.

Tuberculosis

Some research has suggested selenium supplementation, along with other nutrients, can help prevent the recurrence of tuberculosis.

Diabetes

A well-controlled study showed selenium levels are positively correlated with the risk of having type 2 diabetes
Diabetes mellitus type 2
Diabetes mellitus type 2formerly non-insulin-dependent diabetes mellitus or adult-onset diabetesis a metabolic disorder that is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency. Diabetes is often initially managed by increasing exercise and...

. Because high serum selenium levels are positively associated with the prevalence of diabetes, and because selenium deficiency is rare, supplementation is not recommended in well-nourished populations, such as the U.S. More recent studies, however, have indicated selenium may help inhibit the development of type 2 diabetes in men, though the mechanism for the possible preventative effect is not known.

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 colour to glass. This colour cancels out the green or yellow tints that arise from iron impurities that are typical for most glass. For this purpose various salts of selenite and selenate are added. For other application, the red colour is desirable, in which case mixtures of CdSe and CdS are added.

Rubber industry

Small amounts of organoselenium compounds are used to modify the vulcanization catalysts used in the production of rubber.

Uses in the laboratory

Selenium is a catalyst in some chemical reactions but it is not widely used because of issues with toxicity. In X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

, incorporation of one or more Se atoms helps with MAD
Multi-wavelength anomalous dispersion
Multi-wavelength anomalous diffraction is a technique used in X-ray crystallography that facilitates the determination of the three-dimensional structure of biological macromolecules via solution of the phase problem...

 and SAD
Single wavelength anomalous dispersion
Single-wavelength anomalous dispersion is a technique used in X-ray crystallography that facilitates the determination of the structure of proteins or other biological macromolecules by allowing the solution of the phase problem. In contrast to multi-wavelength anomalous dispersion, SAD uses a...

 phasing.

Electronics

The demand for Se by the electronics industry is declining, despite a number of continuing applications. Because of its photovoltaic
Photovoltaics
Photovoltaics is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material...

 and photoconductive
Photoconductivity
Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation....

 properties, selenium is used in photocopying, photocells, light meter
Light meter
A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph...

s and solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....

s. Its use as a photoconductor in plain-paper copiers once was a leading application but in the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers switched to the use of organic photoconductors. It was once widely used in selenium rectifier
Selenium rectifier
A selenium rectifier is a type of metal rectifier, invented in 1933. They were used to replace vacuum tube rectifiers in power supplies for electronic equipment, and in high current battery charger applications....

s. These uses have mostly been replaced by silicon-based devices or are in the process of being replaced. The most notable exception is in power DC surge protection, where the superior energy capabilities of selenium suppressors make them more desirable than metal oxide varistors.

Sheets of amorphous selenium convert x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 images to patterns of charge in xeroradiography
Xeroradiography
Xeroradiography is a type of x-ray in which a picture of the body is recorded on paper rather than on film. In this technique, a plate of selenium, which rests on a thin layer of aluminium oxide, is charged uniformly by passing it in front of a ....

 and in solid-state, flat-panel x-ray cameras.

Zinc selenide
Zinc selenide
Zinc selenide , is a light yellow binary solid compound. It is an intrinsic semiconductor with a band gap of about 2.70 eV at 25 °C. ZnSe rarely occurs in nature...

 is used in blue and white LED
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....

s.

Photography

Selenium is used in the toning of photographic prints
Photographic print toning
In photography, toning is a method of changing the color of black-and-white photographs. In analog photography, toning is a chemical process carried out on silver-based photographic prints. This darkroom process can not be done with a color photograph and although the black-and-white photograph is...

, and it is sold as a toner by numerous photographic manufacturers including Kodak and Fotospeed. Its use intensifies and extends the tonal range of black-and-white photographic images as well as improving the permanence of prints.

Early photographic light meter
Light meter
A light meter is a device used to measure the amount of light. In photography, a light meter is often used to determine the proper exposure for a photograph...

s used selenium
Selenium meter
A selenium meter is a light-measuring instrument based on the photoelectric properties of selenium. The most common use of such light meters is measuring the exposure value for photography...

, but this application is now obsolete.

Medical use

The substance loosely called selenium sulfide (approximate formula SeS2) is the active ingredient in some anti-dandruff shampoos. The selenium compound kills the scalp fungus Malassezia, which causes shedding of dry skin fragments. The ingredient is also used in body lotions to treat Tinea versicolor
Tinea versicolor
Tinea versicolor is a condition characterized by a rash on the trunk and proximal extremities. Recent research has shown that the majority of Tinea versicolor is caused by the Malassezia globosa fungus, although Malassezia furfur is responsible for a small number of cases...

 due to infection by a different species of Malassezia fungus.

Nutrition

Selenium is used widely in vitamin
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...

 preparations and other dietary supplement
Dietary supplement
A dietary supplement, also known as food supplement or nutritional supplement, is a preparation intended to supplement the diet and provide nutrients, such as vitamins, minerals, fiber, fatty acids, or amino acids, that may be missing or may not be consumed in sufficient quantities in a person's diet...

s, in small doses (typically 50 to 200 micrograms per day for adult humans). Some livestock feeds
Fodder
Fodder or animal feed is any agricultural foodstuff used specifically to feed domesticated livestock such as cattle, goats, sheep, horses, chickens and pigs. Most animal feed is from plants but some is of animal origin...

 are fortified with selenium as well.

Detection in biological fluids

Selenium may be measured in blood, plasma, serum or urine to monitor excessive environmental or occupational exposure, confirm a diagnosis of poisoning in hospitalized victims or to assist in a forensic investigation in a case of fatal overdosage. Some analytical techniques are capable of distinguishing organic from inorganic forms of the element. Both organic and inorganic forms of selenium are largely converted to monosaccharide conjugates (selenosugars) in the body prior to being eliminated in the urine. Cancer patients receiving daily oral doses of selenothionine may achieve very high plasma and urine selenium concentrations.

Evolution in biology

Over three billion years ago, blue-green algae were the most primitive oxygenic photosynthetic organisms and are ancestors of multicellular eukaryotic algae. Algae that contain the highest amount of antioxidant selenium, iodide, and peroxidase enzymes were the first living cells to produce poisonous oxygen in the atmosphere. It has been suggested that algal cells required a protective antioxidant action, in which selenium and iodides, through peroxidase enzymes, have had this specific role.. Selenium, which acts synergistically with iodine, is a primitive mineral antioxidant, greatly present in the sea and prokaryotic cells, where it is an essential component of the family of glutathione peroxidase
Glutathione peroxidase
Glutathione peroxidase is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage...

 (GSH-Px) antioxidant enzymes; seaweed
Seaweed
Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...

s accumulate high quantity of selenium and iodine. In 2008, a study showed that iodide also scavenges reactive oxygen species (ROS) in algae, and that its biological role is that of an inorganic antioxidant, the first to be described in a living system, active also in an in vitro assay with the blood cells of today’s humans."

From about three billion years ago, prokaryotic selenoprotein families drive selenocysteine evolution. Selenium is incorporated into several prokaryotic selenoprotein families in bacteria, archaea and eukaryotes as selenocysteine, where selenoprotein peroxiredoxins protect bacterial and eukaryotic cells against oxidative damage. Selenoprotein families of GSH-Px and the deiodinases of eukaryotic cells seem to have a bacterial phylogenetic
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...

 origin. The selenocysteine-containing form occurs in species as diverse as green algae, diatoms, sea urchin, fish and chicken. Selenium enzymes are involved in utilization of the small reducing molecules glutathione
Glutathione
Glutathione is a tripeptide that contains an unusual peptide linkage between the amine group of cysteine and the carboxyl group of the glutamate side-chain...

 and thioredoxin
Thioredoxin
Thioredoxin is a class of small redox proteins known to be present in all organisms. It plays a role in many important biological processes. In humans, it is encoded by the TXN gene. Loss-of-function mutation of either of the two human thioredoxin genes is lethal at the four-cell stage of the...

. One family of selenium-containing molecules (the glutathione peroxidase
Glutathione peroxidase
Glutathione peroxidase is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage...

s) destroy peroxide and repair damaged peroxidized cell membranes, using glutathione. Another selenium-containing enzyme in some plants and in animals (thioredoxin reductase
Thioredoxin reductase
Thioredoxin Reductases are the only known enzymes to reduce thioredoxin . Two classes of thioredoxin reductase have been identified: one class in bacteria and some eukaryotes and one in animals. Both classes are flavoproteins which function as homodimers...

) generates reduced thioredoxin, a dithiol that serves as an electron source for peroxidases and also the important reducing enzyme ribonucleotide reductase
Ribonucleotide reductase
Ribonucleotide reductase is an enzyme that catalyzes the formation of deoxyribonucleotides from ribonucleotides. Deoxyribonucleotides in turn are used in the synthesis of DNA. The reaction catalyzed by RNR is strictly conserved in all living organisms...

 that makes DNA presursors from RNA precursors.

At about 500 Mya, plants and animals began to transfer from the sea to rivers and land, the environmental deficiency of marine mineral antioxidants (as selenium, iodine, etc.) was a challenge to the evolution of terrestrial life. Trace elements involved in GSH-Px and superoxide dismutase enzymes activities, i.e. selenium, vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...

, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, and zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

, may have been lacking in some terrestrial mineral-deficient areas. Marine organisms retained and sometimes expanded their seleno-proteomes, whereas the seleno-proteomes of some terrestrial organisms were reduced or completely lost. These findings suggest that, with the exception of vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s, aquatic life supports selenium utilization, whereas terrestrial habitats lead to reduced use of this trace element. Marine fishes and vertebrate thyroid glands have the highest concentration of selenium and iodine. From about 500 Mya, freshwater and terrestrial plants slowly optimized the production of “new” endogenous antioxidants such as ascorbic acid
Ascorbic acid
Ascorbic acid is a naturally occurring organic compound with antioxidant properties. It is a white solid, but impure samples can appear yellowish. It dissolves well in water to give mildly acidic solutions. Ascorbic acid is one form of vitamin C. The name is derived from a- and scorbutus , the...

 (Vitamin C), polyphenol
Polyphenol
Polyphenols are a structural class of natural, synthetic, and semisynthetic organic chemicals characterized by the presence of large multiples of phenol structural units...

s (including flavonoids), tocopherol
Tocopherol
Tocopherols are a class of chemical compounds of which many have vitamin E activity. It is a series of organic compounds consisting of various methylated phenols...

s, etc. A few of these appeared more recently, in the last 50–200 million years, in fruits and flowers of angiosperm plants. In fact, the angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the late Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 period.

The deiodinase isoenzymes
Isozyme
Isozymes are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes usually display different kinetic parameters Isozymes (also known as isoenzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes...

 constitute another family of eukaryotic selenoproteins with identified enzyme function. Deiodinases are able to extract electrons from iodides, and iodides from iodothyronines. They are, thus, involved in thyroid-hormone regulation, participating in the protection of thyrocytes
Thyroid epithelial cell
Thyroid epithelial cells are cells in the thyroid gland that are responsible for the production and secretion of thyroid hormones, that is, thyroxine and triiodothyronine .-Function:...

from damage by H2O2 produced for thyroid-hormone biosynthesis. About 200 Mya, new selenoproteins were developed as mammalian GSH-Px enzymes.

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