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).