arsenic – selenium – bromine S Se Te       Full table
General
Name, Symbol, Number	selenium, Se, 34
Series	nonmetals
Group, Period, Block	16 (VIA), 4 , p
Density, Hardness	4790 kg/m3(300K), 2
Appearance	grey, metallic lustre
Atomic properties
Atomic weight	78.96 amu
Atomic radius (calc.)	115 (103) pm
Covalent radius	116 pm
van der Waals radius	190 pm
Electron configuration	[Ar]3d104p44s2
e- 's per energy level	2, 8, 18, 6
Oxidation states (Oxide)	±2,4,6 (strong acid)
Crystal structure	hexagonal
Physical properties
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
Miscellaneous
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
iso NA half-life DM DE MeV DP 72Se {syn.} 8.4 d • 0.335 72As 74Se 0.87% Se is stable with 40 neutrons 75Se {syn.} 119.779 d • 0.864 75As 76Se 9.36% Se is stable with 42 neutrons 77Se 7.63% Se is stable with 43 neutrons 78Se 23.78% Se is stable with 44 neutrons 79Se {syn.} 1.13 E6 y ² 0.151 79Br 80Se 49.61% Se is stable with 46 neutrons 82Se 8.73% 1.08 E20 y ² 2.995 82Kr
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.

 

Notable characteristics

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.

Applications

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.

History

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.

Occurrence

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.

Isotopes

Selenium has 28 isotopes, of which 5 are stable.

Precautions

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).

References

• Los Alamos National Laboratory – Selenium (http://periodic.lanl.gov/elements/34.html)

External links

• 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)