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(səlē`nēəm), nonmetallic chemical element; symbol Se; at. no. 34; at. wt. 78.96; m.p. 217°C;; b.p. about 685°C;; sp. gr. 4.81 at 20°C;; valence −2, +4, or +6. Selenium is directly below sulfur in Group 16 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. In chemical activity and physical properties it resembles sulfur and tellurium. Selenium exhibits allotropy, appearing in a number of forms, including a red amorphous powder, a red crystalline material, and a gray crystalline metallike form called "metallic" selenium. A remarkable property (discovered by Willoughby Smith in 1873) of the gray metallic form is that its electrical conductivity is greater in light than in darkness, and it increases as the illumination increases. This property has led to use of the metallic form in the junction rectifier and as a cathode in the photoelectric cell rectifier. Selenium is extensively used in the vulcanization of rubber, in the manufacture of red glass and some enamels, as a decolorizer of glass to counteract the green of iron compounds, in electronics, and in xerography. Selenium forms the oxides SeO2 and SeO3, the selenious (H2SeO3) and selenic (H2SeO4) acids and their respective selenite and selenate salts, a nitride, carbide, hydride, two sulfides, and various halides and oxyhalides. Selenium sometimes occurs in conjunction with sulfur deposits and often occurs as the selenide (especially of copper, lead, silver, and iron) in sulfide ores. Commercially it is obtained chiefly as a byproduct in the refining of copper. In the Great Plains region and certain other areas, selenium is absorbed from the soil by vegetation in quantities sufficient to poison livestock, thus rendering the land useless for grazing. Nonetheless, selenium is one of the elements needed in trace amounts in the animal and human diet. Fish, meat, poultry, whole grains, and dairy products are good sources of this mineral nutrient in the human diet. The element was discovered by Berzelius in 1817.



Se, a chemical element in group VI of Mendeleev’s periodic system. Atomic number, 34; atomic weight, 78.96; primarily a nonmetal. Natural selenium is a mixture of the six stable isotopes 74Se (0.87 percent), 76Se (9.02 percent), 77Se (7.58 percent), 78Se (23.52 percent), 80Se (49.82 percent), and 82Se (9.19 percent). Of the 16 radioisotopes, the most valuable is 75Se, with a half-life of 121 days. The element was discovered in 1817 by J. Berzelius.

Distribution in nature. Selenium is a rare and dispersed element; its content in the earth’s crust (clarke) is 5 X 10–6 percent by weight. The history of selenium in the earth’s crust is closely linked to that of sulfur. Selenium displays a tendency toward concentration and, despite its low clarke, forms 38 minerals of its own, including selenides, selenites, and selenates. Isomor-phous admixtures of selenium are characteristic in sulfides and native sulfur.

Selenium undergoes active migration in the biosphere. Igneous rocks, volcanic gases, and volcanic hot springs are sources of selenium accumulations in organisms. For this reason, the soils and sedimentary rocks in active and dormant volcanic regions are often enriched with selenium. Here, the average content in clays and shales is 6 × 10–5 percent.

Physical and chemical properties. The electronic configuration in the outer subshells of the Se atom is 4s24p4. The spins of the two p electrons are paired, whereas those of the remaining two are unpaired; selenium atoms are therefore able to form Se2 molecules or chains of Sen atoms. Chains of selenium atoms may close to form ring-shaped Se8 molecules. This diverse molecular structure determines the existence of selenium in various allotropic forms. Amorphous (powdery, colloidal, vitreous) and crystalline (monoclinic α and β and hexagonal γ) forms are known. Amorphous (red) powdery and colloidal selenium (density 4.25 g/cm3 at 25°C) are obtained by such methods as the reduction of selenious acid (H2SeO3) from solution and the rapid cooling of selenium vapor. Vitreous (black) selenium (density 4.28 g/cm3 at 25°C) is obtained by heating any form of selenium above 220°C and then rapidly cooling it. Vitreous selenium has a glassy luster and is brittle. Hexagonal (gray) selenium, which exhibits the greatest thermodynamic stability, is obtained from other forms of selenium by heating the selenium to the melting point and then gradually cooling it to 180°–210°C. The selenium is then held at this temperature until crystallization is complete.

The lattice of hexagonal selenium is composed of spiral atomic chains arranged in parallel fashion, and the atoms inside the chains are linked by covalent bonds. The permanent lattices are a = 4.36 angstroms (Å) and c = 4.95 Å. The atomic radius is 1.6 Å; the ionic radius of Se2– is 1.98 Å and of Se4+ is 0.69 Å. Hexagonal selenium has a density of 4.807 g/cm3 at 20°C, a melting point of 217°C, and a boiling point of 685°C. Selenium vapor is yellowish and, when in equilibrium, contains the four polymeric forms SE8 ⇄ Se6 ⇄ Se4 ⇄ Se2, with Se2 predominating at temperatures above 900°C. Hexagonal selenium has a specific heat of 0.19–0.32 kilojoule/kg.°K (0.0463–0.0767 calorie/g°C) in the temperature range –198° to +25°C and 0.34 kilojoule/kg.°K (0.81 calorie/g°C) at 217°C. The thermal conductivity coefficient is 2.344 watts/m.°K (0.0056 calorie/cm.sec.°C). The coefficient of thermal expansion at 20°C for hexagonal single-crystal selenium is 17.88 x 10–6 along the c-axis and 74.09 × 10–6 perpendicular to the c-axis; for polycrystalline selenium, the coefficient is 49.27 ×10–6. Isothermal compressibility β0 = 11.3 X 10–3 per kilobar, and electrical resistivity in the dark at 20°C is equal to 102–

All selenium forms possess photoelectric properties. Up to the melting point, hexagonal selenium is an impurity semiconductor with hole-type conductivity. Selenium is diamagnetic and its vapor is paramagnetic. Selenium is stable upon exposure to air and is unaffected by oxygen, water, hydrochloric acid, and dilute sulfuric acid. It dissolves freely in concentrated nitric acid and aqua regia and dissolves with oxidation in alkalies. Selenium in compounds has the oxidation states – 2, +2, +4, and +6. The ionization potentials for Se0 → Se1+ → Se2+ → S3 + are, respectively, 0.75 electron volt (eV), 21.5 eV, and 32 eV.

Selenium combines with oxygen to form such oxides as SeO, SeO2, and SeO3. The last two are anhydrides of selenious (H2SeO3) and selenic (H2SeO4) acids (the salts are selenites and selenates). SeO2 is the most stable oxide. Selenium combines with halogens to yield such compounds as SeF6, SeF4, SeCl4, SeBr4, and Se2Cl2. Sulfur and tellurium form a continuous series of solid solutions with selenium. Selenium combined with nitrogen yields Se4N4; combined with carbon, it yields CSe2. Compounds with phosphorus, such as P2Se3, P4Se3, and P2Se5, are also known. Hydrogen interacts with selenium at and above 200°C to form H2Se; a solution of H2Se in water is called hydroselenic acid. Selenides are produced upon interaction of selenium with metals. Numerous complex selenium compounds have been obtained. All selenium compounds are toxic.

Production and use. Selenium is obtained from the waste products in the production of sulfuric acid, from wastes of the pulping process in the production of paper, and from the anode slimes of electrolytic copper refineries. Selenium is present in the slimes in association with sulfur, tellurium, and heavy and noble metals. To facilitate selenium extraction, the slime is filtered and subjected to either oxidative roasting at approximately 700°C or heating with concentrated sulfuric acid. The resultant volatile SeO2 is then collected in scrubbers and Cot-trell precipitators. Commercial-grade selenium is precipitated from solutions by sulfur dioxide. Another method of selenium extraction involves the sintering of slime with soda followed by the leaching of sodium selenate with water and the separation of selenium from the solution. To obtain high-purity selenium, which is used as a semiconductor material, crude selenium is refined by such methods as vacuum distillation and recrystallization.

Because of its low cost and reliability, selenium is used in conversion technology in rectifying semiconductor diodes. It is also used in photoelectric cells (hexagonal Se) and in photocopying devices that employ electrostatic processes (amorphous Se). Selenium is required for the synthesis of various selenides. It is also used in thermistors and as a luminophor in television tubes and optical and signaling devices. Selenium is widely used in the clarification of green glass and in the preparation of glass with a reddish color. In metallurgy, selenium gives cast steel a fine-grained structure and improves the mechanical properties of stainless steel. In the chemical industry Se is used as a catalyst. Selenium is also used in the pharmaceutical industry.


In the organism. Most living things contain selenium in tissues in concentrations of 0.01 to 1 mg/kg. Certain microorganisms, fungi, marine organisms, and plants accumulate the element. Certain legumes, for example, milk vetch, Neptunia, and acadia, and members of the families Cruciferae, Rubiaceae, and Compositae accumulate up to 1,000 mg/kg Se (dry weight). Selenium is an essential element for certain plants. Plants that accumulate selenium contain various organic selenium compounds, primarily such selenium analogues of sulfur-bearing amino acids as selenocystathionine, selenohomocysteine, and methyl selenomethionine. Microorganisms which reduce selenites to metallic selenium and oxidize selenides play an important role in the biogenic migration of Se. Biogeochemical provinces of selenium are known to exist.

The human and animal dietary requirement of selenium does not exceed 50–100 micrograms/kg. Selenium has antioxidant properties, increases light reception by the retina, and affects many enzymatic reactions. Acute and chronic poisoning occurs in animals when the selenium content exceeds 2 mg/kg. High selenium concentrations inhibit oxidation-reduction enzymes, disrupt methionine synthesis and the growth of integumentary tissue, and induce anemia. Selenium deficiency in food is accompanied by the onset of the white-muscle disease in animals, necrotic liver degeneration, and exudative diathesis; sodium selenite is administered as a preventive measure against these diseases.



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A highly toxic, nonmetallic element in group 16, symbol Se, atomic number 34; steel-gray color; soluble in carbon disulfide, insoluble in water and alcohol; melts at 217°C; and boils at 690°C; used in analytical chemistry, metallurgy, and photoelectric cells, and as a lube-oil stabilizer and chemicals intermediate.


a nonmetallic element that exists in several allotropic forms. It occurs free in volcanic areas and in sulphide ores, esp pyrite. The common form is a grey crystalline solid that is photoconductive, photovoltaic, and semiconducting: used in photocells, solar cells, and in xerography. Symbol: Se; atomic no.: 34; atomic wt.: 78.96; valency: --2, 4, or 6; relative density: 4.79 (grey); melting pt.: 221°C (grey); boiling pt.: 685°C (grey)