Osmium (Os)

Osmium is a chemical element in the periodic table that has the symbol Os and atomic number 76. A hard brittle blue-gray or blue-black transition metal in the platinum family, osmium is one of the densest natural elements (selecting osmium or iridium as the heaviest element is not possible at this moment) and is used in some alloys with platinum and iridium. The extraordinary density of osmium is a consequence of the lanthanide contraction. Osmium is found native as an alloy in platinum ore and its tetroxide has been used to stain tissues and in fingerprinting. Alloys of osmium are employed in fountain pen tips, electrical contacts and in other applications where extreme durability and hardness are needed.

Notable Characteristics

Osmium in a metallic form is extremely dense, blue white, brittle and lustrous even at high temperatures, but proves to be extremely difficult to make. Powdered osmium is easier to make, but powdered osmium exposed to air leads to the formation of osmium tetroxide (OsO4), which is toxic. The oxide is also a powerful oxidizing agent, emits a strong smell and boils at 130°C.


Due to its very high density osmium is generally considered to be the heaviest known element, narrowly defeating iridium. However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22650 kg/m³ for iridium versus 22610kg/m³ for osmium. Definitive selection between the two is therefore not possible at this time. If one distinguishes different isotopes, then the heaviest ordinary substance would be 192Os.


Osmium has also a very low compressibility. Correspondingly, its bulk modulus is extremely high -- commonly quoted as 462GPa, which is higher than that of diamond but lower than that of ADNR or aggregated diamond nanorods -- although there is some debate in the academic community about whether it is in fact this high. A paper by Cynn et al published in 2002 reported that osmium had this bulk modulus, based on an experimental result; but several subsequent authors have cast doubt upon this (for example, Sahu et al in Sept 2005, who refer also to other such papers).


This metal has the highest melting point and the lowest vapor pressure of the platinum family. Common oxidation states of osmium are +4 and +3, but oxidation states from +1 to +8 are observed.

 

Applications

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 such as osmiridium are very hard and, along with other platinum group metals, is almost entirely used in alloys employed in the tips of fountain pens, phonograph needles, instrument pivots, and electrical contacts, as they can resist wear from frequent use.


Osmium tetroxide has been used in fingerprint detection and in staining fatty tissue for microscope slides. As a strong oxidant, it cross-links lipids by fixing biological membranes in place. Futhermore, osmium atoms are extremely electron dense, making OsO4 an important stain for transmission electron microscopy(TEM) studies of a wide range of biological materials. An alloy of 90% platinum and 10% osmium (90/10) is used in surgical implants such as pacemakers and replacement pulmonary valves.


The tetroxide (and a related compound, potassium osmate) are important oxidants for chemical synthesis, despite being very poisonous.


In 1898 an Austrian chemist - Auer von Welsbach - developed the Oslamp with a filament made of osmium, which he introduced commercially in 1902. After only a few years, osmium was replaced by the more stable metal tungsten (originally known as Wolfram). Tungsten has the highest melting point of any metal, and using it in light bulbs increases the luminous efficacy and life of incandescent lamps.


The light bulb manufacturer OSRAM (founded in 1906 when three German companies; Auer-Gesellerschaft, AEG and Siemens & Halske combined their lamp production facilities), derived its name from the elements of OSmium and wolfRAM.

 

History

Osmium (Greek osme meaning "a smell") was discovered in 1803 by Smithson Tennant in London, England along with iridium in the residue of dissolving platinum in aqua regia.

 

Occurrence

Turkey with 127,000 tons has the world's largest known reserve of osmium. Bulgaria also has substantial reserves of about 2,500 tons. This transition metal is also found in iridiosmium, a naturally occurring alloy of iridium and osmium, and in platinum-bearing river sands in the Ural Mountains, and North and South America. It also occurs in nickel-bearing ores found in the Sudbury, Ontario region with other platinum group metals. Even though the quantity of platinum metals found in these ores is small, the large volume of nickel ores processed makes commercial recovery possible.

 

Value

Osmium is quite valuable, costing about US $100 per gram (g). But one of the stable isotopes, 187Os is worth about $25,000 per gram.

 

Compounds

The chemical compound osmium tetroxide (OsO4), also known as osmium tetraoxide, osmium(VIII) oxide, or osmic acid, is an oxide of the element osmium. The compound is noteworthy for its many uses, despite the rarity of osmium.

 

Isotopes

Osmium has seven naturally-occurring isotopes, 5 of which are stable: 187Os, 188Os, 189Os, 190Os, and (most abundant) 192Os. The other two, 184Os and 186Os, have enormously long half lifes and for practical purposes can be considered to be stable as well. 187Os is the daughter of 187Re (half-life 4.56 x 1010 years) and is most often measured in an 187Os/188Os ratio. This ratio, as well as the 187Re/187Os 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.

 

Precautions

Osmium tetroxide is highly toxic. Airborne low concentrations of osmium can cause lung congestion, skin or eye damage.

 

References

- Los Alamos National Laboratory - Osmium
- National Synchrotron Light Source - Science Highlights
- H Cynn, J E Klepeis, C S Yeo and D A Young, "Osmium has the Lowest Experimentally-Determined Compressibility", Phys. Rev. Lett. 88 #13 (2002).
- B R Sahu and L Kleinman, "Osmium Is Not Harder Than Diamond", Phys. Rev. B 72 (2005).

Original article at http://en.wikipedia.org/wiki/Osmium.