Math Is Fun Forum

Jai Ganesh

Administrator

Registered: 2005-06-28

Posts: 51,538

Selenium

Selenium

Gist

Selenium is used to make pigments for ceramics, paint and plastics. Selenium has both a photovoltaic action (converts light to electricity) and a photoconductive action (electrical resistance decreases with increased illumination). It is therefore useful in photocells, solar cells and photocopiers.

It is found naturally in foods or as a supplement. Selenium is an essential component of various enzymes and proteins, called selenoproteins, that help to make DNA and protect against cell damage and infections; these proteins are also involved in reproduction and the metabolism of thyroid hormones.

Summary

Selenium is a chemical element; it has symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elemental state or as pure ore compounds in Earth's crust. Selenium was discovered in 1817 by Jöns Jacob Berzelius, who noted the similarity of the new element to the previously discovered tellurium (named for the Earth).

Selenium is found in metal sulfide ores, where it substitutes for sulfur. Commercially, selenium is produced as a byproduct in the refining of these ores. Minerals that are pure selenide or selenate compounds are rare. The chief commercial uses for selenium today are glassmaking and pigments. Selenium is a semiconductor and is used in photocells. Applications in electronics, once important, have been mostly replaced with silicon semiconductor devices. Selenium is still used in a few types of DC power surge protectors and one type of fluorescent quantum dot.

Although trace amounts of selenium are necessary for cellular function in many animals, including humans, both elemental selenium and (especially) selenium salts are toxic in even small doses, causing selenosis. Symptoms include (in decreasing order of frequency): diarrhea, fatigue, hair loss, joint pain, nail brittleness or discoloration, nausea, headache, tingling, vomiting, and fever.

Selenium is listed as an ingredient in many multivitamins and other dietary supplements, as well as in infant formula, and is a component of the antioxidant enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) as well as in three deiodinase enzymes. Selenium requirements in plants differ by species, with some plants requiring relatively large amounts and others apparently not requiring any.

Details

Selenium (Se) is a chemical element in the oxygen group (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 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.

Element Properties

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 grams/{cm}^{3}
gray  :  4.79 grams/{cm}^{3}

oxidation states  :  −2, +4, +6.

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 humor is evident in the name he gave the ore, eucairite, meaning “just in time.”

Occurrence and uses

The proportion of selenium in 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 decolorizer; in larger quantities it imparts to glass a clear red color 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.

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

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.

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 colorless, 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.

Additional Information:

Appearance

A semi-metal that can exist in two forms: as a silvery metal or as a red powder.

Uses

The biggest use of selenium is as an additive to glass. Some selenium compounds decolourise glass, while others give a deep red colour. Selenium can also be used to reduce the transmission of sunlight in architectural glass, giving it a bronze tint. Selenium is used to make pigments for ceramics, paint and plastics.

Selenium has both a photovoltaic action (converts light to electricity) and a photoconductive action (electrical resistance decreases with increased illumination). It is therefore useful in photocells, solar cells and photocopiers. It can also convert AC electricity to DC electricity, so is extensively used in rectifiers.

Selenium is toxic to the scalp fungus that causes dandruff so it is used in some anti-dandruff shampoos. Selenium is also used as an additive to make stainless steel.

Biological role

Selenium is an essential trace element for some species, including humans. Our bodies contain about 14 milligrams, and every cell in a human body contains more than a million selenium atoms.

Too little selenium can cause health problems, but too much is also dangerous. In excess it is carcinogenic and teratogenic (disturbs the development of an embryo or foetus).

Natural abundance

Selenium is found in a few rare minerals. Most of the world’s selenium is obtained from the anode muds produced during the electrolytic refining of copper. These muds are either roasted with sodium carbonate or sulfuric acid, or smelted with sodium carbonate to release the selenium.

---

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.