|
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 |
(Gr. osme, a smell) Discovered in 1803 by Tennant in the residue left when crude platinum is dissolved by aqua regia.
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.
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.
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.
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).