Osmium: Densest Precious Metal
Osmium is an extremely hard, brittle, bluish white or gray transition metal in the platinum group metals. It is the densest natural element, being about 0.03 g/cm3 denser than iridium (the second densest natural element) and about twice as dense as lead. Among the platinum family members, osmium has the highest melting point and the lowest vapor pressure.
This precious metal is nearly impossible to fabricate. When alloyed with some of the other metals in the platinum group, such as iridium or platinum, osmium is used in certain applications where extreme hardness and durability are required. A couple examples of these are for electrical contacts and for tips of high quality fountain pens.
Some of the properties osmium possesses are outlined below.
• Chemical Symbol: Os
• Atomic Number: 76
• Category (as an element): Transition Metal
• Group/ Period/ Block (in the Periodic Table): 8/ 6/ d
• Atomic Weight: 190.23 g.mol-1
• Electron Configuration: [Xe] 4f14 5d6 6s2
• Density (near room temperature): 22.59 g.cm-3
• Liquid Density (at melting point): 20 g.cm-3
• Melting Point: 3033°C, 5491°F, 3306°K
• Boiling Point: 5012°C, 9054°F, 5285°K
• Heat of Fusion: 57.85 kJ.mol-1
• Heat of Vaporization: 738 kJ.mol-1
• Oxidation States: 8, 7, 6, 5, 4, 3, 2, 1, 0, -1, -2
• Electronegativity: 2.2 (Pauling scale)
• Atomic Radius: 135 picometre
• Covalent Radius: 144±4 picometre
• Ionization Energies: 840 kJ.mol-1 (first), 1600 kJ.mol-1 (second)
The name "osmium" was derived from the Greek word "osme", which means "smell". It was discovered by the English chemists William Hyde Wollaston and Smithson Tennant in London, England in 1803. Wollaston's and Tennant's discovery of the element involved the discovery as well of the other elements in the platinum group.
With an average mass fraction of 0.05 parts-per notation in the continental crust, osmium is known to be one of the least abundant elements in the Earth's crust. It is found in nature in natural alloys or as a pure element. Similar to the other precious metals in the platinum group, osmium can be found in alloys with copper or nickel.
The extreme toxicity and volatility of osmium's oxide makes it nearly impossible for this element to be used in its pure state. For this reason, it is often necessary to alloy osmium with other elements for use in high-wear applications. For example, osmiridium (a natural alloy of osmium and iridium) is alloyed with the other metals in the platinum group and used in instrument pivots and phonograph needles (apart from electrical contacts and fountain pen tips as mentioned earlier). In another example, osmium tetroxide is used to detect fingerprints and to stain fatty tissue for optical and electron microscopy.
Osmium occurs in the platinum-bearing river sands in North America, South America, and in the Ural Mountains in Russia. The latter, in fact, is known to be the site of the second largest alluvial deposit, which today is still mined. The approximate price of commercial osmium (99 percent pure osmium powder) is 100 U.S. dollars per gram.
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