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History and Production
Derived from Greek Selene, meaning moon. It was discovered by J.J. Berzelius in 1817. Selenium can be obtained as an anode slime deposited during the
electrolytic refining of copper. It has also been obtained from flue dusts remaining from processing copper sulfide ores. Direct extraction of the element from its minerals is not practical because of its rarity.
The use of selenium is in decolorization of glass and when its compound cadmium sulfoselenide is added to glass to give brilliant red. The compound is also used as heat-resistant red pigments in plastics, paints and inks.
The element is important in xenography for dicument copying (xerox machines). It is also used in rectifiers, electronic and solid-state applications.
Physical Data
Selenium is very rare, about 0.05 ppm of the earth's crust, which is much rarer than silver. It can exist in several allotropic forms. Amorphous selenium is red in powder form or black in vitreous form. The latter is the ordinary commercial
form of the element, consists of complex structure of large polymeric rings. The crystalline monoclinic
is a deep red while the hexagonal variety, which is being the most stable form, is metallic gray.
Interatomic distance: 232 pm
Melting point: 221°C (gray)
Boiling point: 685°C (gray)
Thermal conductivity/Wm-1K-1: 2.04 (27°C)
Density/kgm-3: 4790 (gray) (20°C), 3987 (m.p.)
Standard Thermodynamic Data (atomic gas)
Enthalpy of formation: 227.1 kJ/mol
Gibbs free energy of formation: 187 kJ/mol
Entropy: 176.7 J/mol K
Heat capacity: 20.8 J/mol K
Electronic data
Electronic configuration: [Ar] 3d10 4s2 4p4
Term symbol: 4P2
Electron affinity: 194.9652 kJ/mol Electronegativity (Pauline): 2.55
Ionization energy (first, second, third): 940.962, 2044.53, 2973.72 kJ/mol
Chemical properties
References
M. Rayman, 'Se brought to Earh', Chemistry in Britain, Vol. 38, October 2002.
(Impact of Se to health)
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