selenium

/si lee"nee euhm/, n. Chem.

a nonmetallic element chemically resembling sulfur and tellurium, occurring in several allotropic forms, as crystalline and amorphous, and having an electrical resistance that varies under the influence of light. Symbol: Se; at. wt.: 78.96; at. no.: 34; sp. gr.: (gray) 4.80 at 25°C, (red) 4.50 at 25°C.

[ < NL (1818) < Gk selén(e) moon + NL -ium -IUM; named in allusion to its similarity to TELLURIUM]

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Semimetallic chemical element, chemical symbol Se, atomic number 34.

It is widely distributed, usually in small amounts, occasionally uncombined but more often as selenides of iron, lead, silver, or copper. Selenium has several allotropes; the gray metallic crystalline form is the most stable at room temperature. Its electrical conductivity increases when light strikes it, and it can convert light directly into electricity, so selenium is used in photocells (e.g., in light meters and security alarms), solar cells, and photocopiers. It also has been used in rectifiers to convert alternating to direct electric current. It serves as a red colorant for glass and glazes. Selenium has valence 2, 4, and 6 in its compounds, many of which are toxic though the element is not. Selenium dioxide is an important reagent in organic chemistry. Vital to living cells, it works as an antioxidant in the body and is being studied for a variety of possible beneficial health effects; it is used in nutritional supplements and animal feeds.

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▪ chemical element

Introduction

 a chemical element in the oxygen group (oxygen group element) (Group 16 [VIa] of the periodic table), closely allied in chemical and physical properties with the elements sulfur and tellurium. Selenium is rare, composing approximately 90 parts per billion of the crust of the Earth. It is occasionally found uncombined, accompanying native sulfur, but is more often found in combination with heavy metals (copper, mercury, lead, or silver) in a few minerals. The principal commercial source of selenium is as a by-product of copper refining; its major uses are in the manufacture of electronic equipment, in pigments, and in making glass. Selenium is a metalloid (an element intermediate in properties between the metals and the nonmetals). The gray, metallic form of the element is the most stable under ordinary conditions; this form has the unusual property of greatly increasing in electrical conductivity when exposed to light. Selenium compounds are toxic to animals; plants grown in seleniferous soils may concentrate the element and become poisonous.

History

      In 1817 Swedish chemist Jöns Jacob Berzelius noted a red substance resulting from sulfide ores from mines of Falun, Sweden. When this red material was investigated in the following year, it proved to be an element and was named after the Moon or the Moon goddess Selene. An ore of unusually high selenium content was discovered by Berzelius only days before he made his report to the scientific societies of the world on selenium. His sense of humour is evident in the name he gave the ore, eucairite, meaning “just in time.”

Occurrence and uses

      The proportion of selenium in the Earth's crust is about 10⁻⁵ to 10⁻⁶ percent. It has been obtained mainly from the anode slimes (deposits and residual materials from the anode) in electrolytic refining of copper and nickel. Other sources are the flue dusts in copper and lead production and the gases formed in roasting pyrites. Selenium accompanies copper in the refining of that metal: about 40 percent of the selenium present in the original ore may concentrate in copper deposited in electrolytic processes. About 1.5 kilograms of selenium can be obtained from a ton of smelted copper.

      When incorporated in small amounts into glass, selenium serves as a decolourizer; in larger quantities it imparts to glass a clear red colour that is useful in signal lights. The element is also employed in making red enamels for ceramics and steel ware, as well as for the vulcanization of rubber to increase resistance to abrasion.

Allotropy

      The allotropy of selenium is not as extensive as that of sulfur, and the allotropes have not been studied as thoroughly. Only two crystalline varieties of selenium are composed of cyclic Se₈ molecules: designated α and β, both exist as red monoclinic crystals. A gray allotrope having metallic properties is formed by keeping any of the other forms at 200–220 °C and is the most stable under ordinary conditions.

      An amorphous (noncrystalline), red, powdery form of selenium results when a solution of selenious acid or one of its salts is treated with sulfur dioxide. If the solutions are very dilute, extremely fine particles of this variety yield a transparent red colloidal suspension. Clear red glass results from a similar process that occurs when molten glass containing selenites is treated with carbon. A glassy, almost black variety of selenium is formed by rapid cooling of other modifications from temperatures above 200 °C. Conversion of this vitreous form to the red, crystalline allotropes takes place upon heating it above 90 °C or upon keeping it in contact with organic solvents, such as chloroform, ethanol, or benzene.

Preparation

      Pure selenium is obtained from the slimes and sludges formed in producing sulfuric acid. The impure red selenium is dissolved in sulfuric acid in the presence of an oxidizing agent, such as potassium nitrate or certain manganese compounds. Both selenious acid, H₂SeO₃, and selenic acid, H₂SeO₄, are formed and can be leached from residual insoluble material. Other methods utilize oxidation by air (roasting) and heating with sodium carbonate to give soluble sodium selenite, Na₂SeO₃ · 5H₂O, and sodium selenate, Na₂SeO₄. Chlorine may also be employed: its action upon metal selenides produces volatile compounds including selenium dichloride, SeCl₂; selenium tetrachloride, SeCl₄; diselenium dichloride, Se₂Cl₂; and selenium oxychloride, SeOCl₂. In one process, these selenium compounds are converted by water to selenious acid. The selenium is finally recovered by treating the selenious acid with sulfur dioxide.

      Selenium is a common component of ores valued for their content of silver or copper; it becomes concentrated in the slimes deposited during electrolytic purification of the metals. Methods have been developed to separate selenium from these slimes, which also contain some silver and copper. Melting the slime forms silver selenide, Ag₂Se, and copper(I) selenide, Cu₂Se. Treatment of these selenides with hypochlorous acid, HOCl, gives soluble selenites and selenates, which can be reduced with sulfur dioxide. Final purification of selenium is accomplished by repeated distillation.

Physical–electrical properties

      The most outstanding physical property of crystalline selenium is its photoconductivity: on illumination, the electrical conductivity increases more than 1,000-fold. This phenomenon results from the promotion or excitation of relatively loosely held electrons by light to higher energy states (called the conduction levels), permitting electron migration and, thus, electrical conductivity. In contrast the electrons of typical metals are already in conduction levels or bands, able to flow under the influence of an electromotive force.

      The electrical resistivity of selenium varies over a tremendous range, depending upon such variables as the nature of the allotrope, impurities, the method of refining, temperature, and pressure. Most metals are insoluble in selenium, and nonmetallic impurities increase the resistivity.

      Illumination of crystalline selenium for 0.001 second increases its conductivity by a factor of 10 to 15 times. Red light is more effective than light of shorter wavelength.

      Advantage is taken of these photoelectric and photosensitivity properties of selenium in the construction of a variety of devices that can translate variations in light intensity into electric current and thence to visual, magnetic, or mechanical effects. Alarm devices, mechanical opening and closing devices, safety systems, television, sound films, and xerography depend upon the semiconducting property and photosensitivity of selenium. Rectification of alternating electrical current (conversion into direct current) has for years been accomplished by selenium-controlled devices. Many photocell applications using selenium have been replaced by other devices using materials more sensitive, more readily available, and more easily fabricated than selenium.

Compounds

      In its compounds selenium exists in the oxidation states of −2, +4, and +6. It manifests a distinct tendency to form acids in the higher oxidation states. Although the element itself is not poisonous, many of its compounds are exceedingly toxic.

      Selenium combines directly with hydrogen, resulting in hydrogen selenide, H₂Se, a colourless, foul-smelling gas that is a cumulative poison. It also forms selenides with most metals (e.g., aluminum selenide, cadmium selenide, and sodium selenide).

      In combination with oxygen, it occurs as selenium dioxide, SeO₂, a white, solid, chainlike polymeric substance that is an important reagent in organic chemistry. The reaction of this oxide with water produces selenious acid, H₂SeO₃.

      Selenium forms a variety of compounds in which the selenium atom is bonded to both an oxygen and a halogen atom. A notable example is selenium oxychloride, SeO₂Cl₂ (with selenium in the +6 oxidation state), an extremely powerful solvent. The most important acid of selenium is selenic acid, H₂SeO₄, which is as strong as sulfuric acid and more easily reduced.

Robert C. Brasted

atomic number

34

atomic weight

78.96

masses of stable isotopes

74, 76, 77, 78, 80, 82

melting point

amorphous

50 °C (122 °F)

gray

217 °C (423 °F)

boiling point

685 °C (1,265 °F)

density

amorphous

4.28 g/cm³

gray

4.79 g/cm³

oxidation states

−2, +4, +6

electron config.

1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁴

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