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Atomic Number: 34
Atomic Symbol: Se
Atomic Weight: 78.96
Electron Configuration: [Ar]4s23d104p4
History
(Gr. Selene, moon) Discovered by Berzelius in 1817, who found it
associated with tellurium, named for the earth.
Production
Selenium is found in a few rare minerals such as crooksite and
clausthalite. In years past it has been obtained from flue dusts remaining from
processing copper sulfide ores, but the anode metal from electrolytic copper
refineries now provide the source of most of the world's selenium. Selenium is
recovered by roasting the muds with soda or sulfuric acid, or by smelting them
with soda and niter.
Properties
Selenium exists in several allotropic forms. Three are generally
recognized, but as many as that have been claimed. Selenium can be prepared with
either an amorphous or crystalline structure. The color of amorphous selenium is
either red, in powder form, or black, in vitreous form. Crystalline monoclinic
selenium is a deep red; crystalline hexagonal selenium, the most stable variety,
is a metallic gray.
Selenium exhibits both photovoltaic action, where light is
converted directly into electricity, and photoconductive action, where the
electrical resistance decreases with increased illumination. These properties
make selenium useful in the production of photocells and exposure meters for
photographic use, as well as solar cells. Selenium is also able to convert AC
electricity to DC, and is extensively used in rectifiers. Below its melting
point selenium is a p-type semiconductor and is finding many uses in electronic
and solid-state applications.
Elemental selenium has been said to be practically nontoxic and
is considered to be an essential trace element; however, hydrogen selenide and
other selenium compounds are extremely toxic, and resemble arsenic in their
physiological reactions.
Isotopes
Naturally selenium contains six stable isotopes. Fifteen other
isotopes have been characterized. The element is a member of the sulfur family
and resembles sulfur both in its various forms and in its compounds.
Uses
Selenium is used in Xerography for reproducing and copying documents,
letters, etc. It is used by the glass industry to decolorize glass and to make
ruby-colored glasses and enamels. It is also used as a photographic toner, and
as an additive to stainless steel.
Handling
Hydrogen selenide in a concentration of 1.5 ppm is intolerable to
man. Selenium occurs in some solid in amounts sufficient to produce serious
effects on animals feeding on plants, such as locoweed, grown in such soils.
Exposure to selenium compounds (as Se) in air should not exceed 0.2 mg/m3
(8-hour time-weighted average - 40-hour week).
Sources: CRC Handbook of Chemistry and
Physics and the American Chemical Society.
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