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Key People:Jöns Jacob Berzelius...(Show more)Related Topics:Chemical elementoxygen group elementSelenium compound...(Show more)

selenium (Se), a chemical aspect in the oxygen team (Group 16 of the periodic table), closely allied in chemical and physical properties with the facets sulfur and tellurium. Selenium is rare, composing approximately 90 parts per billion of the late of Earth. It is occasionally uncovered uncombined, accompanying indigenous sulfur, but is more often uncovered in combination with hefty metals (copper, mercury, lead, or silver) in a few minerals. The major commercial resource of selenium is as a by-product the copper refining; its major uses are in the manufacture of digital equipment, in pigments, and in make glass. Selenium is a metalloid (an element intermediate in properties in between the metals and also the nonmetals). The gray, metallic kind of the facet is the most stable under plain conditions; this form has the unusual home of considerably increasing in electric conductivity once exposed come light. Selenium compounds room toxic to animals; plants grown in seleniferous soils might concentrate the element and become poisonous.

Element Propertiesatomic numberatomic weightmasses of steady isotopesmelting point amorphous grayboiling pointdensity amorphous grayoxidation stateselectron configuration
74, 76, 77, 78, 80, 82
50 °C (122 °F)
217 °C (423 °F)
685 °C (1,265 °F)
4.28 grams/cm3
4.79 grams/cm3
−2, +4, +6


In 1817 sweden chemist Jöns Jacob Berzelius noted a red substance result from sulfide ores indigenous mines the Falun, Sweden. When this red product was investigated in the following year, it showed to it is in an element and also was called after the Moon or the Moon goddess Selene. An ore the unusually high selenium contents was discovered by Berzelius only days prior to he do his report to the scientific societies that the human being on selenium. His feeling of humour is apparent in the name he gave the ore, eucairite, meaning “just in time.”


Occurrence and uses

The ratio of selenium in earth’s crust is around 10−5 come 10−6 percent. It has actually been derived mainly from the anode slimes (deposits and also residual products from the anode) in electrolytic refining that copper and also nickel. Other sources are the flue dusts in copper and lead production and also the gases created in roasting pyrites. Selenium accompanies copper in the refining of that metal: around 40 percent of the selenium existing in the initial ore might concentrate in copper deposit in electrolytic processes. About 1.5 kilograms of selenium deserve to be acquired from a ton of heated copper.

When incorporated in small amounts into glass, selenium serves as a decolourizer; in bigger quantities it imparts come glass a clean red colour that is helpful in signal lights. The element is also employed in make red enamels for ceramics and also steel ware, and also for the vulcanization that rubber to rise resistance to abrasion.

Selenium refinement efforts are greatest in Germany, Japan, Belgium, and Russia.


The allotropy of selenium is not as extensive as the of sulfur, and the allotropes have actually not been studied as thoroughly. Only two crystalline arrays of selenium space composed the cyclic Se8 molecules: designated α and also β, both exist together red monoclinic crystals. A gray allotrope having metallic nature is created by keeping any type of of the other creates at 200–220 °C and also is the most stable under simple conditions.

An amorphous (noncrystalline), red, powdery type of selenium results once a equipment of selenious acid or among its salts is treated v sulfur dioxide. If the solutions are really dilute, exceptionally fine particles of this range 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 range of selenium is formed by rapid cooling the other modifications from temperatures over 200 °C. Counter of this vitreous kind to the red, crystalline allotrope takes ar upon heater it above 90 °C or upon maintaining it in contact with organic solvents, such together chloroform, ethanol, or benzene.


Pure selenium is obtained from the slimes and sludges developed in producing sulfuric acid. The impure red selenium is dissolved in sulfuric mountain in the visibility of an oxidizing agent, such together potassium nitrate or specific manganese compounds. Both selenious acid, H2SeO3, and also selenic acid, H2SeO4, are formed and also can it is in leached native residual insoluble material. Other approaches utilize oxidation by air (roasting) and also heating through sodium carbonate to offer soluble sodium selenite, Na2SeO3·5H2O, and also sodium selenate, Na2SeO4. Chlorine may additionally be employed: its activity upon metal selenides produces volatile compounds consisting of selenium dichloride, SeCl2; selenium tetrachloride, SeCl4; diselenium dichloride, Se2Cl2; and also selenium oxychloride, SeOCl2. In one process, this selenium compounds are converted through water come selenious acid. The selenium is ultimately recovered by treating the selenious acid v sulfur dioxide.

Selenium is a typical component of ores valued for their content of silver or copper; that becomes focused in the slimes deposited during electrolytic purification the the metals. Methods have actually been developed to separate selenium from these slimes, which likewise contain part silver and copper. Melt the slime creates silver selenide, Ag2Se, and copper(I) selenide, Cu2Se. Therapy of these selenides v hypochlorous acid, HOCl, offers soluble selenites and selenates, which deserve to be decreased with sulfur dioxide. Last purification the selenium is accomplished by repeated distillation.

Physical-electrical properties

The most outstanding physical residential or commercial property of crystalline selenium is that photoconductivity: top top illumination, the electric conductivity increases more than 1,000-fold. This phenomenon results from the promo or excitation of reasonably loosely held electrons by light to higher energy states (called the conduction levels), allow electron hike and, thus, electric conductivity. In contrast the electrons of usual metals are already in conduction level or bands, able to circulation under the influence of one electromotive force.

The electric resistivity the selenium varies end a significant range, depending on such variables as the nature of the allotrope, impurities, the method of refining, temperature, and also pressure. Most steels are insoluble in selenium, and nonmetallic impurities boost the resistivity.

Illumination of crystalline selenium for 0.001 2nd increases that conductivity through a variable of 10 come 15 times. Red light is an ext effective 보다 light of much shorter wavelength.

Advantage is taken of these photoelectric and photosensitivity properties of selenium in the construction of a range of tools that have the right to translate sport in irradiate intensity into electrical current and thence come visual, magnetic, or mechanically effects. Alarm devices, mechanically opening and also closing devices, safety systems, television, sound films, and xerography rely upon the semiconducting property and also photosensitivity of selenium. Rectification of alternating electrical current (conversion into direct current) has for years been accomplished by selenium-controlled devices. Countless photocell applications utilizing selenium have actually been changed by other tools using materials an ext sensitive, an ext readily available, and much more easily fabricated than selenium.


In its compound selenium exist in the oxidation says of −2, +4, and also +6. It manifests a distinct tendency to kind acids in the greater oxidation states. Although the element itself is not poisonous, plenty of of that compounds room exceedingly toxic.

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

In mix with oxygen, it occurs as selenium dioxide, SeO2, a white, solid, chainlike polymeric substance the is critical reagent in organic chemistry. The reaction that this oxide with water to produce selenious acid, H2SeO3.

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Selenium creates a range of compound in which the selenium atom is external inspection to both an oxygen and a halogen atom. A notable instance is selenium oxychloride, SeO2Cl2 (with selenium in the +6 oxidation state), one extremely an effective solvent. The most vital acid of selenium is selenic acid, H2SeO4, i beg your pardon is as strong as sulfuric acid and an ext easily reduced.