Osmium Os

Discovery

In 1803 the English chemist Smithson Tennant studied the black residue left behind when crude platinum was dissolved in aqua regia, a mixture of concentrated nitric and hydrochloric acids. Others had taken the residue for graphite, but Tennant found two new elements in it: osmium and iridium.

Osmium and iridium are so close in density that measurements disagreed for a long time about which is heavier. Precise X-ray crystallographic data now indicate that osmium is very slightly denser. Tennant's work on the platinum residue was a landmark in the chemistry of the platinum-group metals: in the same period William Hyde Wollaston found palladium and rhodium, so four of the six platinum metals were identified within about two years.

Origin of the name

The name comes from the Greek osme, meaning "smell", because finely divided osmium slowly oxidises in air to osmium tetroxide, which has a sharp, chlorine-like odour.

Main uses

• Wear-resistant alloys: osmium-iridium alloys (osmiridium) are extremely hard and were used for fountain-pen nib tips, gramophone needles, instrument pivots and electrical contacts.
• Electron microscopy: osmium tetroxide binds to lipids in cell membranes, fixing and staining biological samples.
• Organic synthesis: osmium tetroxide catalyses the addition of two hydroxyl groups to alkenes; an asymmetric version of this reaction was part of the work recognised by the 2001 Nobel Prize in Chemistry.
• History: osmium filaments were used in early-twentieth-century lamps, and the lighting brand OSRAM combines "osmium" and "Wolfram".

Isotopes

Natural osmium has seven isotopes, with osmium-192 the most abundant at about 41%. Osmium-184 and osmium-186 are radioactive with enormously long half-lives. Osmium-187 is produced by the decay of rhenium-187, so osmium isotope ratios are used to date ores and meteorites and to trace the history of the Earth's mantle.

In everyday life

Solid osmium metal is relatively harmless, but osmium tetroxide is volatile and highly toxic: its vapour can stain the cornea and damage eyesight, so laboratories handle it in sealed ampoules inside fume hoods. Osmium is among the scarcest platinum-group metals and is recovered mainly as a by-product of nickel and platinum refining.