Osmium
|
 |
Atomic Number: |
76 |
|
Melting Point: |
3033 ºC |
| Atomic Symbol: |
Os |
|
Boiling Point: |
5012 ºC |
| Atomic Weight: |
190.2
amu |
|
Density: |
22610
kg/m 3
|
| Atomic Radius: |
135 pm
|
|
Oxidation States: |
3, 4 |
| Covalent Radius: |
128 pm |
|
Electron Configuration: |
[Xe]6s24f145d6 |
| van der Waals Radius: |
-- |
|
State of Matter: |
solid |
History
(Gr. osme, a smell) Discovered in 1803 by Tennant in the residue left
when crude platinum
is dissolved by aqua regia.
Properties
The metal is lustrous, bluish white, extremely hard, and brittle even at high
temperatures. It has the highest melting point and the lowest vapor pressure of
the platinum group. The metal is very difficult to fabricate, but the powdered
or spongy metal slowly gives off osmium tetroxide, which as a powerful oxidizing
agent and has a strong smell. The tetroxide is highly toxic, and boils at 130oC.
The measured densities of iridium and osmium seem to indicate that osmium is
slightly more dense than iridium, so osmium has generally been credited with
being the heavier element. Calculations of the density from the space lattice
which may be more reliable for these elements than actual measurements, however,
give a density of 22.65 for
iridium compared
to 22.661 for osmium. Despite this information, no decision has been made as to
which is heavier.
Sources
Osmium occurs in iridosule and in platinum-bearing river sands in the Urals,
North America, and South America. It is also found in the nickel-bearing ores of
Sudbury, Ontario region along with other platinum metals. While the quantity of
platinum metals
in these ores is very small, the large tonnages of processed nickel ores make
commercial recovery possible.
Uses
The tetroxide has been used to detect fingerprints and to stain fatty tissue
for microscope slides. The tetroxide (and a related compound, potassium osmate)
are important oxidants for chemical synthesis. An alloy of 90% platinum and 10%
osmium (90/10) is used in surgical implants such as pacemakers and replacement
pulmonary valves.
Because of the extreme toxicity of its oxide, osmium is rarely used in its
pure state, and is instead often alloyed with other metals that are used in high
wear applications. Osmium alloys are very hard and along with other platinum
group metals is almost entirely used in alloys employed in the tips of fountain
pen tips, instrument pivots, phonograph needles, and electrical contacts.
Isotopes
Osmium has seven naturally-occurring isotopes, 5 of which are
stable: Os-187, Os-188, Os-189, Os-190, and (most abundant)
Os-192. Os-184, Os-186 have absurdly long half-lifes and for
practical purposes can be considered to be stable as well.
Os-187 is the daughter of rhenium-187 and is most often measured
in a Os-187/Os-186 ratio. This ratio, as well as the
Re-187/Os-187 ratio, have been used extensively in dating
terrestrial as well as meteoric rocks. However, the most notable
application of Os in dating has been in conjunction with
iridium, to analyze the layer of shocked quartz along the K-T
boundary that marks the extinction of the dinosaurs 65 million
years ago.
Hazards
Concentrations in air as low as 107
g/m3 can cause
lung congestion, skin damage, or eye damage. Exposure to osmium tetroxide should
not exceed 0.0016 mg/m3
(8-hour time weighted average - 40-hour work week).