|Name, Symbol, Number||selenium, Se, 34|
|Group, Period, Block||16 (VIA), 4 , p|
|Density, Hardness||4790 kg/m3(300K), 2|
|Atomic weight||78.96 amu|
|Atomic radius (calc.)||115 (103) pm|
|Covalent radius||116 pm|
|van der Waals radius||190 pm|
|e- 's per energy level||2, 8, 18, 6|
|Oxidation states (Oxide)||±2,4,6 (strong acid)|
|State of matter||solid (__)|
|Melting point||494 K (430 °F)|
|Boiling point||957.8 K (1265 °F)|
|Molar volume||16.42 ×10-6 m3/mol|
|Heat of vaporization||26.3 kJ/mol|
|Heat of fusion||6.694 kJ/mol|
|Vapor pressure||0.695 Pa at 494 K|
|Speed of sound||3350 m/s at 293.15 K|
|Electronegativity||2.48 (Pauling scale)|
|Specific heat capacity||320 J/(kg*K)|
|Electrical conductivity||1.0E-10 106/(m·ohm)|
|Thermal conductivity||2.04 W/(m*K)|
|1st ionization potential||941 kJ/mol|
|2nd ionization potential||2045 kJ/mol|
|3rd ionization potential||2973.7 kJ/mol|
|4th ionization potential||4144 kJ/mol|
|Most stable isotopes|
|SI units & STP are used except where noted.|
Selenium is a chemical element in the periodic table that has the symbol Se and atomic number 34. This is a toxic nonmetal that is chemically related to sulfur and tellurium. It occurs in several different forms but one of these is a stable gray metallike form that conducts electricity better in the light than in the dark and is used in photocells. This element is found in sulfide ores such as pyrite.
Selenium exists in a number of allotropic forms. In the powdered form, amorphous selenium is red, while the vitreous form is black. Crystalline hexagonal selenium is a metallic gray, while the monoclinic crystal is a deep red color.
It also exhibits a photovoltaic effect, converting light to electricity, and a photoconductive effect, electrical conductance increasing as selenium is exposed to light. Below its melting point, selenium is a p type semiconductor.
Selenium is an essential micronutrient in all known forms of life; it is a component of the unusual amino acid selenocysteine. Because of its photovoltaic and photoconductive properties, selenium is used extensively in electronics, such as photo cells, and solar cells. Selenium is also extensively used in rectifiers.
Selenium is used to remove color from glass, as it will counteract the green color ferrous impurities impart. It also can be used to give a red color to glasses and enamels. Selenium is used to improve the abrasion resistance in vulcanized rubbers. It also finds application in photocopying.
Another use for selenium is the toning of photographs, and is sold by numerous photographic manufacturers including Kodak and Fotospeed. Its artistic use is to intensify and extend the tonal range of black and white photographic images, and it can also be used for increasing the permanence of images.
Selenium (Latin selene meaning "Moon") was discovered in 1817 by Jons Jacob Berzelius who found the element associated with tellurium.
Growth in selenium consumption was driven by the development of new uses, including applications in rubber compounding, steel alloying, and selenium rectifiers. By 1970, selenium in rectifiers had largely been replaced by silicon, but its use as a photoconductor in plain paper copiers had become its leading application. During the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers using organic photoconductors were produced. In 1996, continuing research showed a positive correlation between selenium supplementation and cancer prevention in humans, but widespread direct application of this important finding would not add significantly to demand owing to the small doses required. In the late 1990s, the use of selenium (usually with bismuth) as an additive to plumbing brasses to meet no-lead environmental standards became important.
Selenium occurs as selenide in many sulfide ores, such as those of copper, silver, or lead. It is obtained as a byproduct of the processing of these ores, from the anode mud of copper refineries and the mud from the lead chambers of sulfuric acid plants. These muds can be processed by a number of means to obtain free selenium.
Selenium has 28 isotopes, of which 5 are stable.
While free selenium is nontoxic, many of its compounds are extremely toxic, and have modes of action similar to that of arsenic. Hydrogen selenide and other compounds are very toxic. Plants grown in selenium-rich soils, such as locoweed, can cause serious effects on animals feeding on the plants.
Selenium and health
Selenium is a trace element in humans. It is used in free radical elimination and other antioxidant enzymes, and also plays a role in the functioning of the thyroid gland. Dietary selenium comes from cereals, meat, fish, and eggs. Brazil nuts are a particularly rich source of selenium.
Selenium deficiency in healthy people is relatively rare. It can occur in patients with severely compromised intestinal function, or those undergoing total parenteral nutrition. Alternatively, people dependent on food that is sourced from selenium-deficient soil are also at risk. The recommended dietary allowance for adults is 55 micrograms per day. More than 400 micrograms per day can lead to toxicity (selenosis).
- Los Alamos National Laboratory – Selenium (http://periodic.lanl.gov/elements/34.html)
- WebElements.com – Selenium (http://www.webelements.com/webelements/elements/text/Se/index.html)
- EnvironmentalChemistry.com – Selenium (http://environmentalchemistry.com/yogi/periodic/Se.html)
- National Institutes of Health page on Selenium (http://ods.od.nih.gov/factsheets/selenium.asp)