Osmium
Symbol |
Name |
Atomic Number |
Atomic Weight |
Group Number |
Os |
Osmium |
76 |
190.2 |
8 |
Standard Sate: solid at 298 K
Color: bluish grey
Osmium metal is lustrous, bluish white, extremely hard, and brittle even at high temperatures. It has the highest melting point and lowest vapour pressure of the platinum group.
(Os), chemical element, one of the platinum metals of Group VIII of
the periodic table and the densest naturally occurring element. A
gray-white metal, osmium is very hard, brittle, and difficult to work,
even at high temperatures. Of the platinum metals it has the highest
melting point, so fusing and casting are difficult. Osmium wires were
used for filaments of early incandescent lamps before the introduction
of tungsten. It has been used chiefly as a hardener in alloys of the
platinum metals, though ruthenium has generally replaced it. A hard
alloy of osmium and iridium has been used for tips of fountain pens
and phonograph needles, and osmium tetroxide is used in certain
organic syntheses.
Pure osmium metal does not occur in nature. Though rare, osmium
is found in native alloys with other platinum metals: in siserskite (up to
80 percent), in iridosmine, in aurosmiridium (25 percent), and in slight
amounts in native platinum. The English chemist Smithson Tennant
discovered the element together with iridium in the residues of
platinum ores not soluble in aqua regia. He announced its isolation
(1804) and named it for the unpleasant odour of some of its
compounds.
Of the platinum metals, osmium is the most rapidly attacked by air.
The powdered metal, even at room temperature, exudes the
characteristic odour of the poisonous, volatile tetroxide, OsO4.
Because solutions of OsO4 are reduced to the black dioxide, OsO2,
by some biological materials, it is sometimes used to stain tissues for
microscopic examinations. Osmium exhibits oxidation states from 0
to +8 in its compounds, with the exception of +1; well-characterized
and stable compounds contain the element in +2, +3, +4, +6, and +8
states. Ruthenium is the only other element known to have a valence
of 8. All compounds of osmium are easily reduced or decomposed
by heating to form the free element as a powder or sponge. Natural
osmium consists of a mixture of seven stable isotopes: osmium-184
(0.02 percent), osmium-186 (1.58 percent), osmium-187 (1.6
percent), osmium-188 (13.3 percent), osmium-189 (16.1 percent),
osmium-190 (26.4 percent), osmium-192 (41.0 percent). atomic
number 76 atomic weight 190.2 melting point 3,000 C (5,432 F)
boiling point about 5,000 C (9,032 F) specific gravity 22.48 (20 C)
valence 2, 3, 4, 6, 8 electronic config. 2-8-18-32-14-2 or (Xe)4f
145d66s2
"osmium" Encyclop�dia Britannica Online.
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