An Overview about the Chemical Element Osmium

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Symbol: Os

Atomic Number: 76

Atomic Mass: 190.23 amu (atomic mass units)

Melting Point: 3045.0 C (3318.15 K, 5513.0 F)

Boiling Point: 5027.0 C (5300.15 K, 9080.6 F)

Number of Protons: 76

Number of Electrons: 76

Number of Neutrons: 114

Classification: Transition Metal or Platinum Group Metal (PGM) or Precious Metal

Crystal Structure: Hexagonal

Density @ 293 K: 22.4 grams per cubic centimeter

Color: bluish/white

Osmium was discovered by Smithson Tennant, a British Chemist, in 1803. He had dissolved a sample of crude platinum in aqua regia (a mixture of hydrochloric and nitric acids) to leave a black residue. This residue contained not only osmium but also another, until then, unknown element iridium. The name osmium comes from the Greek word "osme" for smell or odor.

Osmium along with the elements ruthenium, rhodium, palladium, iridium, and platinum form a group known as the Platinum Group Metals or PGM. Of the PGM osmium has the highest melting point as well as the lowest vapor pressure. Solid osmium metal is very difficult to produce but the powdered element can be sintered to form it in a hydrogen atmosphere at 2000C (2273 K, 3632F). In a powdered form osmium will give of osmium tetroxide at room temperature. Osmium tetroxide is a strong smelling oxidizing agent, it is also highly toxic. Less than a microgram of osmium tetroxide in a cubic meter of air can cause lung, skin or eye damage.

There are seven naturally occurring isotopes of osmium five of which are stable while the other two have extremely long half lives. The most abundant isotope accounting for 40.93% of the total is osmium-192 which is stable. The two unstable isotopes, osmium-184 and osmium-186, account for less than 2% of the total abundance of the element. Over thirty unstable isotopes of osmium have been identified with half lives ranging from 1.7 milliseconds to six years.

Osmium is obtained as a by product from the processing of platinum ores. The element has a number of valuable uses.

* Its main use is in the hard alloys used for the production of nibs for fountain and ball point pens, record player needles, electrical contacts and other equipment where resistance to wear is important.

* An alloy containing 90%platinum and 10% osmium is commonly used in the production of medical implants such as pacemakers and replacement valves.

* Osmium tetroxide is used in histology to stain fatty tissue for microscopic examination. It is also used to help reveal fingerprints.

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