Osmium
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Osmium, a lustrous, bluish-white metal, symbol Os. Osmium was discovered in 1803 by the
British chemist Smithson Tennant in the residue left when crude platinum was dissolved by
aqua regia. Its name comes from the Greek word osme, meaning "smell."
Properties
Osmium's atomic number is 76, and its atomic weight is 190.23. Its melting point is 3033 °C
(5491 °F), and its boiling point is 5012 °C (9054 °F). Osmium, a transition-group metal, is one
of the six-member platinum family of precious metals (Group VIII). It is one of the two densest
known elements. (The density of osmium is extremely close to the density of iridium. Each of
these elements is about twice as dense as lead.)
Osmium metal is extremely hard and brittle, even at high temperatures. It has the highest
melting point and is the least volatile of the platinum group. The specific gravity of the solid is
22.57. (See also Transition Elements; Metal.)
The solid metal does not react with air at room temperature, but finely divided osmium oxidizes
in air to produce the poisonous, volatile, and strong-smelling osmium tetroxide (OsO4).
Similarly, the metal is not attacked by nonoxidizing acids, but powdered osmium dissolves in
fuming nitric acid, aqua regia, and alkaline hypochlorite solutions. The most common oxidation
state for osmium is +8, although compounds exist where osmium has valences of 0 through +7.
Occurrence
Osmium is found in platinum-bearing river sands of the Urals, North America, and South
America. It can also be found in osmiridium, also known as iridosmine, a naturally occurring
alloy consisting mainly of osmium and iridium. Osmium also occurs with other platinum metals
in the nickel-bearing ores of the Sudbury region of Ontario, Canada.
Uses
Osmium is used primarily with other metals of the platinum group for producing very hard
alloys for fountain pen tips, phonograph needles, and electrical contacts. OsO4 is used to detect
fingerprints.
Zvi C. Kornblum
Cooper Union
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How to cite this article:
MLA (Modern Language Association) style:
Kornblum, Zvi C. "Osmium." Encyclopedia Americana. 2010. Grolier Online. 24 July. 2010
<http://ea.grolier.com/article?id=0298160-00>.
Chicago Manual of Style:
Kornblum, Zvi C. "Osmium." Encyclopedia Americana. Grolier Online
http://ea.grolier.com/article?id=0298160-00 (accessed July 24, 2010).
APA (American Psychological Association) style:
Kornblum, Z. C. (2010). Osmium. Encyclopedia Americana. Retrieved July 24, 2010, from
Grolier Online http://ea.grolier.com/article?id=0298160-00
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