Osmium – facts and properties

Position in the periodic table

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Occurrence and extraction

Osmium is one of the rarest stable elements in the Earth's crust. It has no deposits of its own but accompanies the other platinum metals, often in natural alloys with iridium called osmiridium. It is obtained as a by-product of platinum production, and the amounts are very small.

Osmium was discovered in 1803 by the English chemist Smithson Tennant in London. He dissolved crude platinum in aqua regia. A black residue was left behind, which until then had been taken for graphite. Tennant realised that it held two new elements, iridium and osmium. He named osmium after the smell that appeared when the residue was treated: Greek osme. The volatile osmium tetroxide was responsible.

Industrially the platinum-metal residue is oxidised, the volatile tetroxide is distilled off in closed plants with strict protective measures, and the osmium is finally reduced to the metal with hydrogen.

Properties and reactions

Osmium is in period 6 and group 8, below ruthenium, which belongs to period 5. Its electron configuration is [Xe] 4f¹⁴ 5d⁶ 6s² and its electronegativity is 2.2. The metal has a density of 22.57 g/cm³, melts at 3033 °C and boils at 5012 °C. A litre of osmium would weigh well over twenty kilograms, more than twenty times as much as a litre of water.

The density has two causes. Osmium atoms are heavy, and they pack very closely in a dense crystal lattice because, thanks to the lanthanide contraction, they are small for their mass. The metal can hardly be compressed.

Compact osmium is stable in air at room temperature. As a fine powder, or when heated, it forms the tetroxide OsO₄ with oxygen. This is a colourless to pale yellow solid that melts at about forty degrees Celsius and evaporates easily. In this compound osmium reaches oxidation number plus eight, one of the highest oxidation numbers anywhere. The vapours have a sharp smell, somewhat like chlorine, and are highly toxic: they strongly irritate the eyes and airways and can damage the cornea and the lungs. The tetroxide is also a powerful oxidising agent and stains organic tissue black. It has no place in school experiments and may be handled only in specialised laboratories.

Uses in everyday life and technology

Pure osmium is used only in small quantities, since it is scarce and hard to work. In the past, alloys of osmium with iridium and other platinum metals were important for extremely hard, wear-resistant small parts. They were found in fountain pen tips, instrument pivots and electrical contacts. Today alloys with iridium and ruthenium are usually chosen instead.

A piece of industrial history is hidden in the name Osram. Around 1900 Carl Auer von Welsbach built light bulbs with filaments of osmium, which as a metal of high melting point seemed well suited at the time. The filaments were expensive and fragile, and tungsten soon replaced them. The brand Osram, registered in 1906, combines the names osmium and wolfram.

One of the most important modern uses is osmium tetroxide in biology. It fixes fatty structures such as cell membranes and stains them at the same time. Because osmium atoms are so heavy, the stained structures scatter electrons strongly and appear with high contrast in an electron microscope. Only very small quantities are handled, under a fume hood. In organic chemistry the tetroxide serves in tiny proportions as a catalyst for turning double bonds into two hydroxyl groups in a controlled way. For his work on catalytic oxidations, including this reaction, Barry Sharpless shared the Nobel Prize in Chemistry in 2001.

Quick question

What is the atomic number of osmium (Os)?

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