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SELENIUM
- For other uses, see Selenium (disambiguation)
- SE redirects here. For other uses of the abbreviation, see SE. For the ancient Chinese string instrument, see Se (instrument).
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| General |
| Name, Symbol, Number |
selenium, Se, 34 |
| Chemical series |
nonmetals |
| Group, Period, Block |
16, 4, p |
| Appearance |
gray, metallic luster
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| Atomic mass |
78.96(3) g/mol |
| Electron configuration |
[Ar] 3d10 4s2 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 |
| 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 |
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| Atomic properties |
| Crystal structure |
hexagonal |
| Oxidation states |
±2, 4, 6
(strongly acidic oxide) |
| Electronegativity |
2.55 (Pauling scale) |
Ionization energies
(more) |
1st: 941.0 kJ·mol−1 |
| 2nd: 2045 kJ·mol−1 |
| 3rd: 2973.7 kJ·mol−1 |
| Atomic radius |
115 pm |
| Atomic radius (calc.) |
103 pm |
| Covalent radius |
116 pm |
| Van der Waals radius |
190 pm |
| Miscellaneous |
| 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 |
| Notable isotopes |
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| References |
Selenium is a chemical element in the periodic table that has the symbol Se and atomic number 34. Selenium is not found in the free state in nature. It is a nonmetal that is chemically related to sulfur and tellurium. It is toxic in large amounts, but trace amounts of it, forming the active center of certain enzymes, are necessary for the function of all cells in (probably) all living organisms.
Isolated selenium occurs in several different forms but one of these is a stable gray metallike form that conducts electricity better in the light than in the dark and is used in photocells. This element is found in sulfide ores such as pyrite.
Occurrence
Selenium occurs in a number of inorganic forms, including elemental selenium, selenide, selenate and selenite. In soils, selenium most often occurs in soluble forms like selenate (analogous to sulfate), which are leached into rivers very easily by runoff. Selenium also occurs in organic compounds such as dimethyl selenide, selenomethionine and selenocysteine.
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[1].
See also Selenide minerals.
Isotopes
Selenium has at least 29 isotopes, of which 5 are stable, and 6 are nuclear isomers.
History
Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns Jakob Berzelius who found the element associated with tellurium.
Growth in selenium consumption was driven by the development of new uses, including applications in rubber compounding, steel alloying, and selenium rectifiers. By 1970, selenium in rectifiers had largely been replaced by silicon, but its use as a photoconductor in plain paper copiers had become its leading application. During the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers using organic photoconductors were produced. 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.
Selenium and health
Although it is toxic in large doses, selenium is an essential micronutrient in all known forms of life. It is a component of the unusual amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient which functions as cofactor for reduction of antioxidant enzymes such as glutathione peroxidases and thioredoxin reductase. It also plays a role in the functioning of the thyroid gland by participating as a cofactor for thyroid hormone deiodinases [2]. Dietary selenium comes from cereals, meat, fish, and eggs. The recommended dietary allowance for adults is 55 micrograms per day. Liver and Brazil nuts are particularly rich sources of selenium. A list of selenium rich foods can be found on The Office of Dietary Supplements Selenium Fact Sheet.
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[3]. Symptoms of selenosis include a garlic odour 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[4].
- Elemental selenium and most metallic selenides have relatively low toxicities due to their low bioavailability. By contrast, selenate and selenite are very toxic, and have modes of action similar to that of arsenic. Hydrogen selenide is an extremely toxic, corrosive gas[5]. Selenium also occurs in organic compounds such as dimethyl selenide, selenomethionine and selenocysteine, all of which have high bioavailability and are toxic in large doses.
- Selenium poisoning as a result of new agricultural runoff through normally dry lands, with associated sudden leaching of natural soluble selenium which was then concentrated in new "wetlands," has been found to have caused birth defects in wetland birds. [6]
Deficiency
- Selenium deficiency is relatively rare in healthy well-nourished individuals. It can occur in patients with severely compromised intestinal function, or those undergoing total parenteral nutrition. Alternatively, people dependent on food grown from selenium-deficient soil are also at risk. The Dietary Reference Intake for adults is 55 micrograms per day.
- Selenium deficiency can lead to Keshan disease, which is potentially fatal, and contributes (along with iodine deficiency) to Kashin-Beck disease[7]. The primary symptom of Keshan disease is myocardial necrosis, leading to weakening of the heart. Kashin-Beck disease results in atrophy, degeneration and necrosis of cartilage tissue [8]. Keshan disease also makes the body more susceptible to illness caused by other nutritional, biochemical, or infectious diseases. These diseases are most common in certain parts of China where the soil is extremely deficient in selenium. Studies in Jiangsu Province of China have indicated a reduction in the prevalence of these diseases by taking selenium supplements. Selenium deficiency has also been associated with goitre, cretinism and recurrent miscarriage in humans[9].
Controversial Health Effects
Cancer
- Several studies have suggested a link between cancer and selenium deficiency[10][11][12][13][14][15][16]. A study conducted on the effect of selenium suplementation on the recurrence of skin cancers did not demonstrate a reduced rate of reccurence of skin cancers, but did show a significantly reduced occurrence of total cancers[17]. 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 long-term study of selenium levels in over 60,000 participants did not show any correlation between selenium levels and risk of cancer[18]. The SU.VI.MAX study [19] 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 31% 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 [20]. The SELECT study is currently investigating the effect of selenium and vitamin E supplementation on incidence of prostate cancer.
HIV/AIDS
- 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[21] or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated.
- This thesis is further weakened by the fact that AIDS is a phenomenon that is only 30 years old, whereas the communities in sub-Saharan Africa have been in existence for millions of years. If selenium levels were that important a factor in the AIDS epidemic, one would logically argue that AIDS would have long been a presence there. Furthermore, AIDS has been particularly prominent in a few sub-Saharan countries with a vast copper mining infrastructure, notably the Democratic Republic of Congo and Zambia -- Selenium is a byproduct of copper mining -- which further invalidates the thesis that low selenium levels have anything to do with the spread of AIDS. Lastly, there has been no quantifiable definition, or threshold of what "low selenium levels" represents, which makes the idea of making a comparison impossible to verify.
- Low selenium levels in AIDS patients have been directly correlated with decreased immune cell count and increased disease progression and risk of death[22]. 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[23].
- Regardless of the cause of depleted selenium levels in AIDS patients, studies have shown that selenium deficiency does strongly corelate with the progression of the disease and the risk of death[24][25][26]. Selenium supplementation may help mitigate the symptoms of AIDS and reduce the risk of mortality. It should be emphasized that the evidence to date does not suggest that selenium can reduce the risk of infection or the rate of spread of AIDS, but rather treat the symptoms of those who are already infected.
Production
Selenium is a common byproduct of copper refining, or the production of sulfuric acid[27][28][29]. Isolation of selenium is often complicated by the presence of other compounds and elements. Commonly, production begins by oxidation with sodium carbonate to produce sodium selenite. The sodium selenite is then acidified with sulfuric acid producing selenous acid. The selenous acid is finally bubbled with sulfur dioxide producing elemental selenium.
Non biological applications
- Used in photocells, light meters and solar cells because of its photovoltaic and photoconductive properties.
- Used in rectifiers (this is a classic use which has since been largely replaced by silicon diodes).
- Used to remove color from glass, as it will counteract the green color that ferrous impurities impart.
- Used to give a red color to glasses and enamels.
- Used to improve the abrasion resistance in vulcanized rubbers.
- Used in photocopying.
- The toning of photographs (in printing); sold as a toner by numerous photographic manufacturers including Kodak and Fotospeed.
- Artistic use intensifies and extends the tonal range of black and white photographic images,
- Use increases the permanence of print photographic images.
In popular culture
- See selenium's entries at fictional applications of real materials.
Compounds
See also Selenium compounds and organoselenium chemistry.
External links
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