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Selenium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                34              arsenic ← selenium → bromine
                 S
                ↑
                Se
                ↓
                Te

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                     Name, Symbol, Number selenium, Se, 34
                                                 Chemical series nonmetals
                                             Group, Period, Block 16, 4, p
                                    Appearance gray-black, metallic luster
                                               Atomic mass 78.96 (3) g/mol
                               Electron configuration [Ar] 3d^10 4s^2 4p^4
                                           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

   CAPTION: 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 ±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
                                                         Selected isotopes

                 CAPTION: 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%  Se is stable with 40 neutrons
                      ^75Se syn    119.779 d    ε      -             ^75As
                                                γ      0.264, 0.136,
                                                       0.279         -
                      ^76Se 9.36%  Se is stable with 42 neutrons
                      ^77Se 7.63%  Se is stable with 43 neutrons
                      ^78Se 23.78% Se is stable with 44 neutrons
                      ^79Se syn    1.13×10^6 y  β^-    0.151         ^79Br
                      ^80Se 49.61% Se is stable with 46 neutrons
                      ^82Se 8.73%  1.08×10^20 y β^-β^- 2.995         ^82Kr

                                                                References

   Elemental selenium in different allotropic forms: black, gray, and red
   Enlarge
   Elemental selenium in different allotropic forms: black, gray, and red

   Selenium ( IPA: /səˈliːniəm/) is a chemical element with atomic number
   34, with the chemical symbol Se. 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 centre of certain enzymes, are necessary for the
   function of all cells in (probably) all living organisms.

   Isolated selenium occurs in several different forms, but the most
   stable of these is a dense gray semimetal (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
   allotropic section below). Selenium also exists in many nonconductive
   forms: a black glass-like substance, as well as several red crystalline
   forms built of eight-membered ring molecules, like its lighter cousin
   sulfur.

   Selenium is found in highest quantities in sulfide ores such as pyrite.

Occurrence

   Selenium occurs naturally in a number of inorganic forms, 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 in biology also occurs in organic compounds such as dimethyl
   selenide, selenomethionine and selenocysteine. In these compounds
   selenium plays an analogous role to 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 .

Isotopes

   Selenium has at least 28 isotopes, of which 5 are stable, and 6 are
   nuclear isomers.

History and changing global demand for selenium

   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).

   Growth in selenium consumption was historically driven by steady
   development of new uses, including applications in rubber compounding,
   steel alloying, and selenium rectifiers. By 1970, selenium in
   rectifiers had largely been replaced by silicon, but its use as a
   photoconductor in plain paper copiers had become its leading
   application. During the 1980s, the photoconductor application declined
   (although it was still a large end-use) as more and more copiers using
   organic photoconductors were produced. Presently, the largest use of
   selenium world-wide is in glass manufacturing, followed by uses in
   chemicals and pigments. Electronic use, despite a number of continued
   applications, continues to decline.

   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.

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 . 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 . 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 .

   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 .
   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 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 it evaporates. High selenium levels produced in this
   fashion have been found to have caused certain birth defects in wetland
   birds.

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. Selenium deficiency also contributes (along with iodine
   deficiency) to Kashin-Beck disease . 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 . 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 .

Controversial Health Effects

Cancer

          Several studies have suggested a link between cancer and
          selenium deficiency . A study conducted on the effect of
          selenium supplementation 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
          . 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 . The
          SU.VI.MAX study 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 . The SELECT study is currently investigating
          the effect of selenium and vitamin E supplementation on
          incidence of prostate cancer. However, selenium has been proved
          to 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 . One of the study
          showed that in just 72 hours, the efficacy of treatment using
          chemotherapeutic drugs, such as Taxol and Adriamycin, with
          selenium yeast is significantly higher than the treatment using
          the drugs alone. The finding was shown in various cancer cells
          (breast, lung, small intestine, colon, liver).

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 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, 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 .

          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 . 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 and allotropic forms

   Selenium is a common byproduct of copper refining, or the production of
   sulfuric acid . 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 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
   Se[8] molecules, similar to many allotropes of sulfur.

Non biological applications

   Glass and ceramic use (largest use world-wide)
     * Used to give a red colour to glasses and enamels
     * Used to remove colour from glass, as it will counteract the green
       colour that ferrous impurities impart.

   Chemical use
     * Selenium is a catalyst in many chemical reactions and is widely
       used in various industrial and laboratory syntheses.

   Manufacturing and materials use
     * Used with bismuth in brasses to replace more-toxic lead.
     * Used to improve the abrasion resistance in vulcanized rubbers.

   Electronic use
     * Used in photocopying
     * Used in photocells, light meters and solar cells because of its
       photovoltaic and photoconductive properties.
     * Once widely used in rectifiers

   All these uses have been replaced by silicon-based devices, or are in
   the process of being replaced.

   Photographic uses
     * Used in 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.

   All photographic uses rapidly declining in the modern era of digital
   photography and digital photographic printing.

   Biological use
     * Used widely in vitamins and food supplements, in small doses
       (typically 50 to 200 micrograms per day for adult humans. Some
       livestock feeds are fortified as well).

In popular culture

          See selenium's entries at fictional applications of real
          materials.

Compounds

     * Mercury selenide (HgSe)
     * Hydrogen selenide (H[2]Se)
     * Selenium dioxide (SeO[2])
     * Selenic acid (H[2]SeO[4])
     * Selenous acid (H[2]SeO[3])
     * Selenium sulfides: Se[4]S[4], SeS[2], Se[2]S[6]
     * Sodium selenite (Na[2]SeO[3])
     * Zinc selenide (ZnSe)

   Retrieved from " http://en.wikipedia.org/wiki/Selenium"
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   with only minor checks and changes (see www.wikipedia.org for details
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