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Elements sulfur and tellurium share chemical and physical properties with selenium, an element of the oxygen group (Group 16 of the periodic table).There are 90 parts per billion of selenium in the crust of the Earth, which makes it rare.Some minerals contain it uncombined, along with native sulfur, but others combine it with heavy metals (copper, mercury, lead, or silver) in small amounts.The principal source of selenium is a by-product of copper refining; its major applications are in electronic equipment, pigments, and glassmaking.Selenium is a metalloid (an element with properties intermediate between a metal and a nonmetal).The gray, metallic form of the element is the most stable under normal conditions. When exposed to light, this form of the element increases in electrical conductivity.It is toxic for animals to consume selenium compounds; plants grown in seleniferous soil may accumulate the element and turn poisonous.

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


chemist Jöns Jacob Berzelius noted that sulfuric ores from Falun mines resulted in a red substance.In the following year, an investigation proved that this red material was an element. It was named Selene, after the Moon goddess.He discovered an ore containing unusually high levels of selenium only days before he made his report to the scientific societies of the world.A humorous name, eucairite, he gave to the ore, means "just in time.".


Occurrence and uses

.Generally, anode slimes are obtained from the anode (deposits or residuals from the anode) in electrolytic copper or nickel refining.Flue dusts in copper and lead production and pyrite roasting can also provide sources. .A tonne of smelted copper can provide 1.5 kilograms of selenium.

.A red enamel can also be made using this element, as well as red rubber to increase resistance to abrasion.

There is a strong effort to refine selenium in Germany, Japan, Belgium, and Russia.


There are not as many allotropes of sulfur as there are of selenium, and fewer studies have been made of them.Among all the different types of selenium crystals, only two are composed of cyclic Se8 molecules: designated * and *, both occur as red monoclinic crystals.Keeping any of the other forms at 200–220 °C for 20 minutes produces the most stable gray allotrope, which is characterized by metallic properties.

.Diluted solutions of this variety of particles will produce a transparent red colloidal suspension.During the carbonization of molten selenite glass, clear red glass results.By rapidly cooling selenium from temperatures above 200 °C, a glassy, almost black variety is formed.The vitreous form can be converted into the red, crystalline allotropes upon heating above 90 °C or by being kept in contact with organic solvents, such as chloroform, ethanol, or benzene.


In the production of sulfuric acid, slimes and sludges contain pure selenium.In the presence of an oxidizing agent, such as potassium nitrate or some manganese compounds, impure red selenium is dissolved in sulfuric acid.It is possible to leach selenous acid, H2SeO3, and selenic acid, H2SeO4, from residual insoluble material.In other methods, sodium selenite is oxidized by air (roasted) and heated with sodium carbonate to produce soluble sodium selenate, Na2SeO3·5H2O, and sodium selenate, Na2SeO4. .Solenious acid is produced when water converts these selenium compounds into selenium dioxide.Selenium is then recovered after the sulfur dioxide is treated with selenious acid.

Selenium is common in ores valued for their silver or copper content; it becomes concentrated in slimes deposited during the electrolytic purification of metals.These slimes also contain silver and copper, so methods were developed to separate the selenium from them.Upon melting the slime, silver selenide, Ag2Se, and copper(I) selenide, Cu2Se, are formed.With the addition of hypochlorous acid, HOCl, these selenides become soluble selenites and selenates, which are then reduced with sulfur dioxide.The final purification of selenium is accomplished by repeated distillation.

Physical-electrical properties

Photoconductivity is the most spectacular characteristic of crystalline selenium: its power to conduct electricity increases 1000-fold at illumination. .Typical metals, by contrast, already have electrons in conduction levels or bands, ready to flow in response to an electromotive force.

There is a wide range of electrical resistivity of selenium, which depends, for example, on its allotrope, impurities, the refining method, temperature, and pressure.Selenium is highly resistive due to the insoluble nature of many metals and the presence of nonmetallic impurities.

Its conductivity increases by between 10 and 15 times over 0.001 seconds of illumination of crystalline selenium.The shorter wavelength red light has greater efficiency.

Selenium's photoelectric and photosensitivity properties are used in the construction of many devices that respond visually, magnetically, and mechanically to changes in light intensity.Selenium is used in alarm devices, mechanical opening and closing devices, safety systems, television, sound films, and xerography because of its photosensitivity and semiconducting property.Several devices controlled by selenium have been used to rectify alternating electric current (turn it into direct current).There are now many photocells that use materials that are more sensitive, easier to fabricate, and more readily available than selenium for some applications.


Selenium exists as the oxidation states *2, +4, and +6 in its compounds.At higher oxidation states, it forms acids more readily.Even though the element itself is not poisonous, many of its compounds are highly toxic.

Hydrogen selenide, or H2Se, is formed when selenium reacts directly with hydrogen. It is a colourless, foul-smelling gas that accumulates to poison.In addition to aluminum selenide, cadmium selenide and sodium selenide, it forms selenides with most other metals.

SeO2 is synthesized from selenium and oxygen and is inorganic chemistry's most important reagent.By reacting this oxide with water, selenious acid is produced, H2SeO3.

.Selenium oxychloride, SeO2Cl2 (with selenium in its +6 oxidation state), is a notable example of this type of solvent.Selenium's main acid is seleninic acid, H2SeO4, which is as strong as sulfuric acid and more easily reduced.