|Name, symbol, number||selenium, Se, 34|
|Group, period, block||16, 4, p|
|Standard atomic weight||78.96 g·mol−1|
|Electron configuration||[Ar] 4s2 3d10 4p4|
|Electrons per shell||2, 8, 18, 6 (Image)|
|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|
|Oxidation states||6, 4, 2, 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||120 pm|
|Covalent radius||120±4 pm|
|Van der Waals radius||190 pm|
|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|
|Brinell hardness||736 MPa|
|CAS registry number||7782-49-2|
|Most stable isotopes|
|Main article: Isotopes of selenium|
Selenium (pronounced /sɨˈliːniəm/ sə-LEE-nee-əm) 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.
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 section Allotropes 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 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 are rare. The chief commercial uses for selenium today are in glassmaking and in chemicals and pigments. Uses in electronics, once important, have been supplanted by silicon semiconductor devices.
Selenium salts are toxic in large amounts, but trace amounts of the element 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.
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 sulfuric acid 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. 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 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 (see SECIS element).
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.
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 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 plants. Anthropogenic sources of selenium include coal burning and the mining and smelting of sulfide ores.
See also Selenide minerals.
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.
Industrial production of selenium often involves the extraction of selenium dioxide from residues obtained during the purification of copper. Commonly, production begins by oxidation with sodium carbonate to produce selenium dioxide. The selenium dioxide is then mixed with water and the solution is acidified to form selenous acid (oxidation step). Selenous acid is bubbled with sulfur dioxide (reduction step) to give elemental selenium.
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 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 (see figure). 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.
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), which, for practical purposes, can be considered to be stable. Twenty-three other unstable isotopes have been characterized.
See also Selenium-79 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.
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). 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 that remove reactive oxygen species such as peroxide:
Selenium also plays a role in the functioning of the thyroid gland, by participating as a cofactor for the three known thyroid hormone deiodinases. It may inhibit Hashimotos's 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 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). In descending order of concentration, high levels are also found in kidney, tuna, crab, and lobster.
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 Astragalus species (including some locoweeds), prince's plume (Stanleya sp.), woody asters (Xylorhiza sp.), and false goldenweed (Oonopsis sp.)
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. 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. 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. The Chinese people that 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. A dose of selenium as small as 5 mg per day can be lethal for many humans.
Symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage. Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary edema, and death. Elemental selenium and most metallic selenides have relatively low toxicities 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. Hydrogen selenide is an extremely toxic, corrosive gas. 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.
On April 19, 2009, twenty-one polo 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 compound, with which the horses had been injected. Such vitamin injections are common to promote recovery after a match. The pharmacy did not initially release the name of the specific ingredient due to ongoing law-enforcement and other investigations. Analysis of inorganic compounds of the vitamin supplement indicated that selenium concentrations were ten to fifteen times higher than normal in the horses' blood samples and 15 to 20 times higher than normal in their liver samples. It was later confirmed that selenium was the ingredient in question.
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.
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 on advanced-aged people (over 90). Also, people dependent on food grown from selenium-deficient soil are also at risk. However, although New Zealand 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 nutrient such as iodine and vitamin E. The interaction is observed in the etiology of many deficiency diseases in animals and pure selenium deficiency is in fact rare. The effect of selenium deficiency on health remains uncertain, particularly in relation to Kashin-Beck disease.
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, 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 that selenium and vitamin E supplementation, both alone and together, did not significantly reduce the incidence of prostate cancer in 35,000 men who "generally were replete in selenium at baseline". The SELECT trial found that 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 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 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. 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. 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.
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 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 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”.
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 T-cell associated genes encode selenoproteins similar to human glutathione peroxidase. 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.
Some research has suggested that selenium supplementation, along with other nutrients, can help prevent the recurrence of tuberculosis.
A well-controlled study showed that selenium intake is positively correlated with the risk of developing type 2 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.
Experimental findings have demonstrated a protective effect of selenium on methylmercury toxicity, but epidemiological studies have been inconclusive in linking selenium to protection against the adverse effects of methylmercury.
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.)
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.
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.
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.
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 due to infection by a different species of Malassezia fungus.
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.
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. Venturi et al. 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 antioxidant enzymes (GSH-Px); seaweeds accumulate high quantity of selenium and iodine. In 2008, Küpper et al. 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 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.
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, magnesium, copper, and zinc, 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 vertebrates, 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 (Vitamin C), polyphenols (including 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. About 200 Mya, new selenoproteins were developed as mammalian GSH-Px enzymes.
Selenium occurs in the 0, +2, +4, +6 and −2 valence states.
SELENIUM [[[symbol]] Se, atomic weight 79.2 (0 =16)], a nonmetallic chemical element, discovered in 1817 by J. J. Berzelius, who called it selenium (Gr. o X v?, the moon) on account of its close analogy with tellurium (Lat. tellus, the earth). It is occasionally found in the native condition, but more frequently in combination with metals in the form of selenides, the more important seleniferous minerals being euchairite, crookesite, clausthalite, naumannite and zorgite. It is also found as a constituent of various pyrites and galenas, and in some specimens of native sulphur. The element is usually obtained from the flue dust or chamber deposits of sulphuric-acid works in which a seleniferous pyrites is burned. In this process, the residues are boiled with a dilute sulphuric acid to which nitric acid and potassium chlorate are added in order to transform the element into selenic acid, H 2 Se0 4, which is then reduced to selenious acid, H 2 Se0 3, by boiling with hydrochloric acid, and finally to selenium by sulphur dioxide. L. F. Nilson (Ber., 1874, 7, p. 1719) digests the well-washed chamber mud with a moderately concentrated solution of potassium cyanide, whereby the element goes into solution in the form of potassium selenocyanide, KSe(CN), from which it is precipitated by hydrochloric acid. As alternative methods, F. Wehler (Ann., 1859, 109, p. 375) heats the well-washed chamber residues with potassium nitrate and carbonate in order to obtain an alkaline selenate, which is then boiled with hydrochloric acid, yielding selenious acid, from which the element is obtained as above; whilst H. Rose (Pogg. Ann., 1828, 90, p. 471) by the action of chlorine obtains selenium tetrachloride, which is converted into selenious acid by water, and the acid so prepared is finally reduced to selenium by treatment with sodium sulphite (see also G. Magnus, Pogg. Ann., 1830, 96, p. 165; 0. Pettersson, Ber., 1873, 6, p. 1 477; H. Koch, German Patent 167457, 1903). It is obtained from zorgite by heating the mineral with aqua regia; the excess of acid is evaporated, and the resulting syrupy liquid diluted, filtered and decomposed by sulphur dioxide, when the selenium is precipitated (Billandot, Ency. chimique, 1883, 5, p. 198).
The commercial element usually contains a certain amount of sulphur, and some tellurium, and various methods have been devised for its purification. L. Oppenheim (Jour. prakt. Chem., 18 57, 71, p. 2 79) fuses the commercial selenium with potassium cyanide in a stream of hydrogen, takes up the melt in water and passes air through the solution; the precipitated tellurium is filtered off, and the solution then supersaturated with hydrochloric acid, when selenium is gradually deposited. E. Divers, (Chem. News, 1885, 5 1, p. 199) dissolves the element in boiling concentrated sulphuric acid and reduces the resulting selenious acid with sulphur dioxide, filters off the precipitate and washes it with water and alcohol. The resulting product, however, still contains traces of sulphur. C. Hugot (Ann. chim. phys., 1900 (7), 21, p. 34) converts the element by dilute nitric acid into selenium dioxide which is then sublimed, and dissolved in water. Any sulphuric acid present is removed by baryta water, the precipitated barium sulphate filtered off, the solution acidified by hydrochloric acid and reduced by sulphur dioxide.
Several allotropic forms of selenium have been described, but the work of A. P. Saunders (Jour. Phys. Chem., 1900, 4, p. 423) seems to establish that the element exists in three distinct forms, namely liquid selenium (which includes the vitreous, soluble and amorphous forms), crystalline red selenium (which includes, perhaps, two very closely allied forms), and crystalline, grey or metallic selenium. Liquid selenium becomes more and more viscous in character as its temperature falls from 220° C. to 60° C.; it is soft at about 60°, but is hard and brittle between 30° and 40°. It shows a conchoidal fracture. The amorphous variety, which only differs from the vitreous form in its state of aggregation, is obtained by reducing solutions of selenious acid with sulphur dioxide. It is slightly soluble in carbon bisulphide. The red crystalline variety is obtained by crystallization of selenium from carbon bisulphide, or by leaving the amorphous form in contact with the same solvent. The grey crystalline form is obtained by heating the other varieties, and is the most stable form from ordinary temperatures up to 217°. All varieties of selenium dissolve in concentrated sulphuric acid, forming a green solution (see also R. Marc, Ber., 1906, 39, p. 697; and W. Oechsner de Coninck, Comptes rendus, 1906, 143, p. 682). A colloidal selenium was obtained by C. Paal and C. Koch (Ber., 1905, 38, p. 526) by reducing selenious acid dissolved in an aqueous solution of sodium protalbate with hydrazine hydrate and hydrochloric acid, the precipitate obtained being then dissolved in sodium carbonate. The specific gravity of selenium is 4.8; the specific heat varies from 0.0716 to 0.1147, depending upon the particular form. Selenium combines directly with hydrogen when heated in the gas, and with fluorine in the cold. It burns with a blue flame when heated in the air or in oxygen, at the same time giving a characteristic smell of rotten horseradish, a reaction which serves for the recognition of the element. It combines directly with nitrogen, phosphorus, antimony and carbon, and with all the metals (except gold) to form selenides, of which those of the alkali and alkaline earth metals are soluble in water. Metallic selenium is a conductor of electricity, and its conductivity is increased by light; this property has been utilized in apparatus for transmitting photographs by telegraphy (see Telegraph).
Seleniuretted Hydrogen, H 2 Se, is obtained by the direct union of its constituent elements in the heat; by the decomposition of various selenides with mineral acids; by the decomposition of aluminium selenide, or phosphorus selenide with water; by the action of selenium on a concentrated solution of hydriodic acid; and by heating selenium with colophene (H. Moissan), or better with paraffin wax (H. Wuyts and A. Stewart, Bull. Soc. chim. Belg., 1909, 23, p. 9). It is a colourless gas which possesses a characteristic smell, more unpleasant than sulphuretted hydrogen. Its physiological effects are much more persistent and injurious than sulphuretted hydrogen, producing temporary paralysis of the olfactory nerves and inflammation of the mucous membrane. It may be liquefied, the liquid boiling at41° to - 42° C. and becoming solid at - 68° C. (K. Olszewski). It is somewhat soluble in water and forms a hydrate. It is decomposed by heat, burns with a blue flame, and behaves as a reducing agent. It precipitates many of the heavy metals as selenides when passed into solutions of their salts. Its aqueous solution is unstable, gradually depositing red selenium on standing. Selenium fluoride, SeF4, is obtained as a colourless liquid by the direct action of fluorine or selenium (P. Lebeau, Comptes rendus, 1907, 144, p. 1042). It boils at about 100° C., attacks glass readily, is decomposed by water, and dissolves iodine. Selenium dichloride, Se 2 C1 21 is obtained by the action of chlorine on selenium; by the action of phosphorus pentachloride on selenium or the dioxide; by the action of hydrochloric acid on seleno-sulphur trioxide (E. Divers, Chem. News, 1884, 49, p. 212): 2S Se03+2HC1=H2S04+ S Se0 3. SeC1 2 (+H 2 O)-->Se 2 C1 2 -1-SO 2 (OH)Cl; and by heating selenium and selenium tetrachloride to ioo° C. in a sealed tube. It is a yellowish-brown oily liquid which commences to distil at 130° C. with partial decomposition into selenium and the tetrachloride. It is decomposed by water with formation of selenium and selenious acid: 2Se 2 C1 2 +3H 2 0 = H2Se03-1-3Se Cl. Selenium tetrachloride, SeCl 4, is obtained by passing excess of chlorine over selenium; by the action of phosphorus pentachloride on selenium dioxide: Se0 2 +PC1 5 =SeOC1 2 +POC1 3 i 3SeOC12-I-2POC13=3SeC14-1-P205; and by the action of thionyl chloride on selenium oxychoride. It is a white solid which can be obtained crystalline by sublimation in a current of chlorine. It dissociates when heated, and is decomposed by water with production of selenious acid. It dissolves selenium. Similar bromides and iodides are known. Selenyl chloride, SeOCl 21 is formed when selenium tetrachloride is heated with the dioxide to 150° C. (R. Weber, Pogg. Ann., 1859, 184, p. 615), or when the dioxide is heated with common salt. 2Se02+2NaC1=SeOC12-1-Na2Se03. It is a yellow-coloured liquid which solidifies at o° C., and fumes on exposure to air. It combines with titanium and tin bichlorides and with antimony trichloride, and it is decomposed by water.
Selenium dioxide, Se02, is prepared by burning selenium in oxygen, or by oxidizing selenium with nitric acid and heating the residue. It may also be prepared by the action of selenium on sulphur oxyfluoride (H. Moissan, Bull. Soc. chim., 1902 (3), 27, p. 251): 2S0 2 F 2 +Se +S10 2 = Se0 2 +2502+SiF4. It crystallizes in needles or prisms and volatilizes when heated, giving a pale yellow vapour. It is very hygroscopic, and dissolves in water and alcohol. It reacts with the caustic alkalis to form selenites, and combines directly with hydrocyanic acid. It is decomposed by hydriodic acid with liberation of selenium and iodine, and by ammonia with formation of selenium and nitrogen. Selenious acid, H 2 SeO 3, is obtained in the crystalline form when a solution of selenium dioxide in water is concentrated over sulphuric acid. It effloresces on exposure to air. Oxidizing agents readily convert it into selenic acid, whilst reducing agents transform it into selenium. It yields normal, acid and super-acid salts (e.g. KHSe0 3 H 2 5e0 3) It is decomposed by many acids with liberation of selenium. Selenic acid, H2Se04, was discovered by E. Mitscherlich (Pogg. Ann., 1827, 85, p. 623). Its salts, the selenates, are obtained by the oxidation of the selenites, and the free acid may be obtained by the decomposition of the lead or barium salt. It is also obtained in the electrolysis of solutions of selenious acid (C. Manuelli and G. Lazzarini, Gazz., 1909, 39, 1, p. 50). The acid crystallizes in hexagonal prisms and melts at 58° C. It dissolves in water and yields a hydrate of composition H 2 SeO 4 H 2 O. It is very hygroscopic, dissolves sulphur readily and acts on organic compounds in a manner similar to sulphuric acid. It decomposes when strongly heated. The selenates are isomorphous with the chromates and sulphates. A compound of selenium and sulphur has been described as resulting from the action of sulphuretted hydrogen on selenious acid, but A. Gutbier (Zeit. anorg. Chem., 1905, 43, p. 384) is of the opinion that in this reaction, at ordinary temperature, a simple reduction takes place, leading to the formation of a mixture of sulphur and selenium. Selenium sulphoxide, SeS0 3, is formed as a yellowish crystalline mass when selenium is warmed with sulphur trioxide. It decomposes when heated above 35° C., and also in the presence of water. A compound of composition, SeS0 5, has been obtained by the addition of selenium dioxide to sulphuric acid saturated with sulphur trioxide (R. Metznen, Ann. chim. phys., 1898, (7), 15, p. 203). It crystallizes in colourless needles. Selenosulphuric acid, H 2 SeS0 3, is only known in the form of its salts, which are usually obtained by the action of selenium on solutions of the metallic sulphites, a selenotrithionate being simultaneously produced. The salts are unstable and readily decompose when heated. Selenotrithionic acid, H2Se5206, is also obtained in the form of its potassium salt by the action of potassium hydrogen sulphite on a selenosulphate. It is readily decomposed by acids with liberation of sulphur dioxide and selenium.
Nitrogen selenide, N2Se2, is formed by the decomposition of selenium chloride with ammonia (A. Verneuil, Bull. soc. chim., 1882, 38, p. 54 8). It crystallizes readily from benzene or acetic acid and explodes when subjected to shock or when heated. It is also obtained when dry ammonia gas is passed into a dilute solution of selenyl chloride in benzene, the precipitate produced being digested with potassium cyanide to remove any selenium (V. Lenher and E. Wolesensky, Jour. Amer. Chem. Soc., 1907, 29, p. 215). It is a brickred powder which explodes when heated to 130° C. Selenium cyanide, Se(CN) 2, is obtained by decomposing silver selenocyanide with cyanogen iodide, or by the action of silver cyanide on a solution of selenium bromide in carbon bisulphide. It crystallizes in tables and is very soluble in water. A more complex cyanide, Se3(CN)2, is obtained by passing a current of chlorine and air into an aqueous solution of potassium selenocyanide (A. Verneuil, Ann. chim. phys., 1886 (6), 9, p. 289). It crystallizes in golden yellow needles and is decomposed by boiling water: 2Se3(CN)2+2H20=4HCN-I-Se02+ 5Se. When heated to 180° C. in vacuo it yields the simple cyanide Se(CN) 2. Potassium selenocyanide, KSeCN, is obtained by the action of selenium on a concentrated aqueous solution of potassium cyanide, or by heating selenium with anhydrous potassium ferrocyanide (W. Crookes, Ann., 1851, 78, p. 177). It crystallizes in needles, possesses an alkaline reaction, and is readily decomposed by acids with liberation of selenium. It forms numerous double salts.
Numerous determinations of the atomic weight of selenium have been made. The earlier results of J. J. Berzelius from an analysis of the chloride gave values from 79.2 to 79.35. Later determinations by V. Lenher (Jour. Amer. Chem. Soc., 1898, 20, p. 595), from the analysis of silver selenite and the reduction of the double selenium ammonium bromide, give values from 79 2 77 to 79.3 6 7; whilst J. Meyer (Ber., 1902, 35, p. 1 59 1) by the electrolysis of silver selenite in the presence of potassium cyanide obtained the value 79.22.
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For etymology and more information refer to: http://elements.vanderkrogt.net/elem/se.html (A lot of the translations were taken from that site with permission from the author)
Selenium (selenium atoms only) has several different forms. They are called allotropes. The most stable of these is a dense gray semimetal (an element that is partially a metal and a nonmetal). The way the atoms are put together is a named a trigonal polymer chain. It lets electricity pass through it better in the light than in the dark. This form is used in photocells.
Selenium also has many nonconductive forms: a black glass-like substance, as well as several red crystalline forms. In the red form the eight atoms of selenium form ring to make a molecule. These rings then stack together to make the solid red selenium. The way that red seleniums atoms are put together is similar to sulfur.
Selenium is not very reactive. It does not dissolve in acids. It reacts with air when powdered to make selenium(IV) oxide. It does not react with many things, so selenium(IV) oxide is what most selenium compounds are made from.
Selenium forms several oxidation states; -2, +2, +4, and +6. The -2 state is in selenides. Selenides are strong reducing agents. They are stronger reducing agents than sulfides. Hydrogen selenide is the acid made from selenide ion. Selenium also reacts with reactive metals to make selenides, such as sodium selenide or aluminium selenide.
The +1 state is found in some selenium compounds, such as selenium(I) chloride. They are the most unreactive selenium compounds.
The +4 state is found in selenites. Selenites and selenous acid are moderate oxidizing agents. It is made by dissolving selenium dioxide in water and making selenous acid. Then selenous acid is reacted with bases to make selenites.
The +6 state is found in selenates. Selenates and selenic acid are powerful oxidizing agents. Selenic acid can even dissolve gold! They are made by reacting hydrogen peroxide with selenium(IV) oxide to get selenium(VI) oxide, which dissolves in water to make selenic acid. Selenates are more reactive than sulfates.
Most selenium compounds are colorless. Selenium compounds are not common.
Sodium selenite in a bottle
Selenium is very rarely found as an element in the ground. Most selenium in soil is in a very tiny amount. It is easily washed away. It is in very small amounts in the human body. Selenium is found most in sulfide ores like pyrite. This selenium is in selenides. Selenium is gotten as a byproduct. Sometimes selenium is concentrated in plants. It can also be leached into rivers when copper is mined. Too much selenium is bad for a river. Some coal has selenium in it.
It is made as a byproduct when refining copper and certain other sulfide ores. Selenium is made by oxidizing selenide ores to selenium dioxide. The selenium dioxide is dissolved in acidic water to make selenous acid, which is reacted with sulfur dioxide to make selenium as an element. It is the red form that is made. To make the black form, the red form is heated and melted.
Selenium is used in photocells. The gray metallic form is used, as it changes its electrical conductivity when light shines on it. Selenium is also used as a catalyst. The largest use is to color glass red. It can be used in special brasses instead of lead. It is used in some rectifiers. Most use silicon instead. It is used to tone photographs. It can be used to remove dandruff from hair in the form of selenium sulfide.
Selenium is a trace element in the human body. Humans need very small amounts of selenium. Only about 50-200 micrograms are needed. Selenium can be toxic if more than 400 micrograms are taken. Once there was coal that had a large amount of selenium in it. People were getting selenium poisoning from the coal.
Selenium deficiency is rare. Although selenium has some helpful effects on the human body, it also has some harmful effects and only a very little should be eaten.
It is toxic in large amounts. Some selenium compounds are very toxic and harmful to things that live in water. Eating 5 mg of selenium a day can kill you after a while.