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34 arsenicseleniumbromine


Name, Symbol, Number selenium, Se, 34
Element category nonmetals
Group, Period, Block 16, 4, p
Appearance gray-black, metallic luster
Standard atomic weight 78.96(3)  g·mol−1
Electron configuration [Ar] 4s2 3d10 4p4
Electrons per shell 2, 8, 18, 6
Physical properties
Phase solid
Density (near r.t.) (gray) 4.81  g·cm-3
Density (near r.t.) (alpha) 4.39  g·cm-3
Density (near r.t.) (vitreous) 4.28  g·cm-3
Liquid density at m.p. 3.99  g·cm−3
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−1
Heat of vaporization 95.48  kJ·mol−1
Specific heat capacity (25 °C) 25.363  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 500 552 617 704 813 958
Atomic properties
Crystal structure hexagonal
Oxidation states 6, 4, 2, 1,[1] -2
(strongly acidic oxide)
Electronegativity 2.55 (Pauling scale)
Ionization energies
1st:  941.0  kJ·mol−1
2nd:  2045  kJ·mol−1
3rd:  2973.7  kJ·mol−1
Atomic radius 115pm
Atomic radius (calc.) 103  pm
Covalent radius 116  pm
Van der Waals radius 190 pm
Magnetic ordering no data
Thermal conductivity (300 K) (amorphous)
0.519  W·m−1·K−1
Thermal expansion (25 °C) (amorphous)
37  µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 3350 m/s
Young's modulus 10  GPa
Shear modulus 3.7  GPa
Bulk modulus 8.3  GPa
Poisson ratio 0.33
Mohs hardness 2.0
Brinell hardness 736  MPa
CAS registry number 7782-49-2
Most-stable isotopes
Main article: Isotopes of selenium
iso NA half-life DM DE (MeV) DP
72Se syn 8.4 d ε - 72As
γ 0.046 -
74Se 0.87% 74Se is stable with 40 neutrons
75Se syn 119.779 d ε - 75As
γ 0.264, 0.136,
76Se 9.36% 76Se is stable with 42 neutrons
77Se 7.63% 77Se is stable with 43 neutrons
78Se 23.78% 78Se is stable with 44 neutrons
79Se syn 2.95×105 y β- 0.151 79Br
80Se 49.61% 80Se is stable with 46 neutrons
82Se 8.73% 1.08×1020 y β-β- 2.995 82Kr

Selenium (pronounced /səˈliːniəm/) is a chemical element with the atomic number 34, represented by the chemical symbol Se, an atomic mass of 78.96. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature. It is toxic in large amounts, but trace amounts of it are necessary for cellular function in most, if not all, animals, forming the active center of the enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) and three known deiodinase enzymes (which convert one thyroid hormone to another). Selenium requirements in plants differ by species, with some plants apparently requiring none.[2]

Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semi-metal (semiconductor) form that is structurally a trigonal polymer chain. It conducts electricity better in the light than in the dark, and is used in photocells (see allotropes section below). Selenium also exists in many non-conductive forms: a black glass-like allotrope, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter chemical cousin sulfur.

Selenium is found in economic quantities in sulfide ores such as pyrite, partially replacing the sulfur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but all are rare.


[edit] Occurrence

Selenium occurs naturally in a number of inorganic forms, including selenide, selenate and selenite. In soils, selenium most often occurs in soluble forms such as selenate (analogous to sulfate), which are leached into rivers very easily by runoff.

Selenium has a biological role, and it is found in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine. In these compounds selenium plays a role analogous to that of sulfur.

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, 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 free selenium.

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.[3]

See also Selenide minerals.

[edit] Isotopes

Selenium has six naturally occurring isotopes, five of which are stable: 74Se, 76Se, 77Se, 78Se, and 80Se. The last three also occur as fission products, along with 79Se which has a half-life of 295,000 years. The final naturally occurring isotope, 82Se, has a very long half-life (~1020 yr, decaying via double beta decay to 82Kr) and which for practical purposes can be considered to be stable. 23 other unstable isotopes have been characterized.

See also Selenium-79 for more information on recent changes in the half-life of this fission product, important for the dose calculations performed in the frame of the geological disposal of long-lived radioactive waste.

[edit] History and global demand

Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns Jakob Berzelius who found the element associated with tellurium (named for the Earth). It was discovered as a byproduct of sulphuric acid production.

It came to medical notice later because its toxicity to humans working in industry. It was also recognised as an important veterinary toxin. In 1954 first hints towards 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 enzymes. This was followed by the discovery of selenocysteine in proteins. During the 1980s it was shown that selenocystine was encoded by the codon TGA. The recoding mechanism was worked out first in bacteria and then in mammals.

Growth in selenium consumption was historically driven by steady development of new uses, including applications in rubber compounding, steel alloying, and selenium rectifiers. Selenium is also an essential material in the drums of laser printers and copiers. 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 more and more copiers using organic photoconductors were produced. Currently, the largest use of selenium worldwide is in glass manufacturing, followed by uses in chemicals and pigments. Electronics use, despite a number of continued applications, continues to decline.[4]

In 1996, continuing research showed a positive correlation between selenium supplementation and cancer prevention in humans, but widespread direct application of this important finding would 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 world selenium production continues to increase modestly.

[edit] Health effects and nutrition

Although it is toxic in large doses, selenium is an essential micronutrient for animals. In plants, it occurs as a bystander mineral, sometimes in toxic proportions in forage (some plants may accumulate selenium as a defense against being eaten by animals, but other plants such as locoweed require selenium, and their growth indicates the presence of selenium in soil).[2] It is a component of the unusual amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient which functions as cofactor for reduction of antioxidant enzymes such as glutathione peroxidases and certain forms of thioredoxin reductase found in animals and some plants (this enzyme occurs in all living organisms, but not all forms of it in plants require selenium).

Glutathione peroxidase (GSH-Px) catalyzes certain reactions which remove reactive oxygen species such as peroxide:

2 GSH+ H2O2---------GSH-Px → GSSG + 2 H2O

Selenium also plays a role in the functioning of the thyroid gland by participating as a cofactor for the three known thyroid hormone deiodinases.[5]

Dietary selenium comes from nuts, cereals, meat, fish, and eggs. Brazil nuts 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). High levels are found in kidney, tuna, crab and lobster, in that order.[6][7]

[edit] Selenium indicator plants

Certain species of plants are considered indicators of high selenium content of the soil, since they require high levels of selenium in order to thrive. The main selenium indicator plants are locoweed (Astragalus species), prince's plume (Stanleya sp.), woody asters (Xylorhiza sp.), and false goldenweed (Oonopsis sp.)[8]

[edit] Toxicity

Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis.[9] This 400 microgram Tolerable Upper Intake Level is primarily based 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.[10] 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, but also to avoid creating an imbalance of nutrients in the diet and to account for data from other countries.[11] The Chinese people who suffered from selenium toxicity ingested selenium by eating corn grown in extremely selenium-rich stony coal (carbonaceous shale). This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded in literature.[12]

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.[13] Elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability. By contrast, selenates and selenites are very toxic, having an oxidant mode of action similar to that of arsenic trioxide. The chronic toxic dose of selenite for human beings is about 2400 to 3000 micrograms of selenium per day for a long time.[14] Hydrogen selenide is an extremely toxic, corrosive gas.[15] Selenium also occurs in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine, all of which have high bioavailability and are toxic in large doses. Nano-size selenium has equal efficacy, but much lower toxicity.[16][17][18][19][20][21][22]

Selenium poisoning 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.[23]

[edit] Deficiency

Selenium deficiency is relatively rare in healthy, well-nourished individuals. It can occur in patients with severely compromised intestinal function, those undergoing total parenteral nutrition, and also[24] on advanced-aged people (over 90). Alternatively, people dependent on food grown from selenium-deficient soil are also at risk.

[edit] Controversial health effects

Several studies have suggested a possible link between cancer and selenium deficiency,[25][26][27][28] In 2009 the 5.5 year SELECT study reported that selenium and vitamin E supplementation, both alone and together, did not significantly reduce the incidence of prostate cancer in 35,000 well-nourished men.[29] One earlier 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, although without a statistically significant change in overall mortality.[30] The preventative effect was greatest in those with the lowest baseline selenium levels.[29] The SELECT trial found that vitamin E did not reduce prostrate 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.[29]
Dietary selenium prevents chemically induced carcinogenesis in many rodent studies.[31] It has been proposed that selenium may help prevent cancer by acting as an antioxidant 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.[32] The SU.VI.MAX study[33] concluded that 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.[34] However, there is evidence that 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.[35] Studies of cancer cells in vitro showed that chemotherapeutic drugs, such as Taxol and Adriamycin, were more toxic to strains of cancer cells grown in culture when selenium was added.[36] [37]
In March 2009, a study from the Department of Cancer Biology at the University of Texas M. D. Anderson Cancer Center reports that Vitamin E (400 IU) and selenium (200 micrograms) supplements affect gene expression and can act as a tumor suppressor.[38] Eric Klein, MD from the Glickman Urological and Kidney Institute in Ohio said the new study “lend credence to the previous evidence that selenium and vitamin E might be active as cancer preventatives”.[39] In an attempt to rationalise the differences between epidemiological and in vitro studies and randomised trials like SELECT, Klein said that 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”.[40]
Some research has indicated a geographical link between regions of selenium-deficient soils and peak incidences of HIV/AIDS infection. For example, much of sub-Saharan Africa is low in selenium. However, Senegal 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[41] 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.[42] Selenium normally acts as an antioxidant, so low levels of it may increase oxidative stress on the immune system leading to more rapid decline of the immune system. Others have argued that HIV encodes for the human selenoenzyme glutathione peroxidase, which depletes the victim's selenium levels. Depleted selenium levels in turn lead to a decline in CD4 helper T-cells, further weakening the immune system.[43]
Regardless of the cause of depleted selenium levels in AIDS patients, studies have shown that selenium deficiency does strongly correlate with the progression of the disease and the risk of death.[44][45][46]
Some research has suggested that selenium supplementation, along with other nutrients, can help prevent the recurrence of tuberculosis.[47]
A well-controlled study showed that selenium intake is positively correlated with the risk of developing type II diabetes. 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.[48]
Experimental findings have demonstrated an interaction between selenium and methylmercury, but epidemiological studies have found little evidence that selenium helps to protect against the adverse effects of methylmercury.[49]

[edit] Production and allotropic forms

Selenium is a common byproduct of copper refining, or the production of sulfuric acid.

[50][51][52] Isolation of selenium is often complicated by the presence of other compounds and elements. Commonly, production begins by oxidation with sodium carbonate to produce selenium dioxide. The selenium dioxide is then mixed with water producing selenous acid. The selenous acid is finally bubbled with sulfur dioxide producing elemental red amorphous selenium.

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 most thermodynamically stable and dense form of selenium is the electrically conductive gray (trigonal) form, which is composed of long helical chains of selenium atoms. The conductivity of this form is notably light sensitive. Selenium also exists in three different deep-red crystalline monoclinic forms, which are composed of Se8 molecules, similar to many allotropes of sulfur.[53]

[edit] Non-biologic applications

Selenium is a catalyst in many chemical reactions and is widely used in various industrial and laboratory syntheses, especially Organoselenium chemistry. It is also widely used in structure determination of proteins and nucleic acids by X-ray crystallography (incorporation of one or more Se atoms helps with MAD and SAD phasing.)
Manufacturing and materials use
The largest use of selenium worldwide is in glass and ceramic manufacturing, where it is used to give a red color to glasses, enamels and glazes as well as to remove color from glass by counteracting the green tint imparted by ferrous impurities.
Selenium is used with bismuth in brasses to replace more toxic lead. It is also used to improve abrasion resistance in vulcanized rubbers.
Because of its photovoltaic and photoconductive properties, selenium is used in photocopying, photocells, light meters and solar cells. It was once widely used in rectifiers. 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 images to patterns of charge in xeroradiography and in solid-state, flat-panel x-ray cameras.
Selenium is used in the toning of photographic prints, 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.

[edit] Biologic applications

Medical use
The substance loosely called selenium sulfide, SeS2, actually selenium disulfide or selenium (IV) sulfide, is the active ingredient in some dandruff shampoos.[54] The effect of the active ingredient is to kill the scalp fungus Malassezia which causes shedding of dry skin fragments. The ingredient is also used in body lotions to treat Tinea versicolor due to infection by a different species of Malassezia fungus.[55]
Selenium is used widely in vitamin preparations and other dietary supplements, in small doses (typically 50 to 200 micrograms per day for adult humans). Some livestock feeds are fortified with selenium as well.

[edit] 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.[56] 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. Venturi et al.[56][57] 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,[58] 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 antioxidant enzymes (GSH-Px). In fact, seaweeds accumulate high quantity of selenium and iodine.[56] In 2008, Küpper et al.,[59] 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 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,[60] where selenoprotein peroxiredoxins protect bacterial and eukaryotic cells against oxidative damage. Selenoprotein families of GSH-Px and deiodinase of eukaryotic cells seem to have a bacterial phylogenetic origin. The selenocysteine-containing form occurred in green algae, diatoms, sea urchin, fish and chicken, too. One family of selenium-containing molecules as glutathione peroxidases repairs damaged cell membranes, while another (glutathione S-transferases) repairs damaged DNA and prevents mutations.[61]

When 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.[56] Trace elements involved in GSH-Px and superoxide dismutase enzymes activities, i.e. selenium, vanadium, magnesium, copper and zinc, may have been lacking in some terrestrial mineral-deficient areas.[60] Marine organisms apparently 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 vertebrates, aquatic life supports selenium utilization, whereas terrestrial habitats lead to reduced use of this trace element.[62] 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 (Vitamin C), polyphenols, flavonoids, tocopherols, 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 period.

The deiodinase isoenzymes constituted the second family of eukaryotic selenoproteins with identified enzyme function. Deiodinases are able to extract electrons from iodides, and iodides from iodothyronines; so, they are involved in thyroid-hormone regulation, participating in the protection of thyrocytes from damage by H2O2 produced for thyroid-hormone biosynthesis.[56][57] About 200 Mya, new selenoproteins were developed as mammalian GSH-Px enzymes.[63][64][65][66]

[edit] Compounds

Selenium occurs in the 0,+2,+4,+6 and -2 valence states. See also Selenium compounds and organoselenium chemistry.

[edit] References

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