Osmium · element 76
The densest element you can dig out of the ground, named after the smell of its oxide
Key facts
Osmium is a hard, brittle metal with a faint blue sheen, and it holds a record that no other natural element can beat: it is the densest of them all. A piece of osmium weighs almost three times as much as the same sized piece of iron and about twice as much as a lump of lead. It belongs to the platinum group, a cluster of six rare, unreactive metals that sit together in the middle of the periodic table.
Its name comes from the Greek word osme, meaning smell. Finely powdered osmium reacts slowly with air to form osmium tetroxide, a volatile compound with a sharp, unpleasant odour. That same compound is the reason chemists treat osmium with respect.
Where it is found
Osmium is one of the rarest stable elements in the Earth's crust and never forms an ore of its own. It turns up as a minor passenger alongside platinum and nickel in a few huge igneous deposits: the Bushveld complex in South Africa, Norilsk in Siberia and the Sudbury basin in Canada. It also occurs as grains of natural osmium-iridium alloy in river gravels.
Almost all commercial osmium is a by-product of refining nickel and platinum ores, and world output is measured in hundreds of kilograms a year rather than tonnes. Geologists value it for a different reason: rhenium-187 decays into osmium-187 over billions of years, giving a natural clock for dating rocks and meteorites.
How it was discovered
In 1803 the English chemist Smithson Tennant, working in London, dissolved crude platinum in aqua regia, a mixture of nitric and hydrochloric acids. Other chemists had noticed that a black residue was left behind and had dismissed it as graphite. Tennant suspected otherwise. By treating the residue with alkalis and acids in turn he teased out two new metals, announcing them to the Royal Society in 1804.
He named one iridium for its rainbow of coloured salts, and the other osmium for the smell of its oxide. Tennant died in 1815 after a riding accident in France, before either metal had found a practical use.
What we use it for
Osmium's hardness and resistance to wear made it useful wherever a tiny surface had to survive endless rubbing: fountain pen nibs, gramophone needles, compass pivots and electrical contacts were once tipped with an osmium-iridium alloy. Today it is mostly added in small amounts to harden platinum alloys.
The compound osmium tetroxide has two important jobs. In biology labs it is used to prepare samples for electron microscopes, because it binds to fatty membranes and its heavy atoms scatter electrons strongly, making cell structures show up dark and crisp. In synthetic chemistry it is the key ingredient in a reaction that adds two hydroxyl groups to a double bond in a controlled way, work that shared the 2001 Nobel Prize in Chemistry. Osmium also had a brief career in early light bulb filaments before tungsten took over.
Reading the numbers
Osmium melts at about 3030 °C and boils at roughly 5010 °C; only a few elements, such as tungsten and rhenium, stay solid at higher temperatures. Its density of 22.59 grams per cubic centimetre means a one litre block would weigh over 22 kilograms, with iridium only a whisker behind. These period 6 metals are so dense because their atoms are barely larger than the lighter metals above them, yet each atom carries far more mass.
The electron configuration [Xe] 4f14 5d6 6s2 shows eight electrons beyond the xenon core in the outer d and s orbitals. Eight outer electrons places osmium in group 8, directly beneath iron and ruthenium. Like iron it can adopt many oxidation states, but osmium goes further, reaching +8 in osmium tetroxide, one of the highest oxidation states any element achieves. A first ionisation energy of 840 kJ/mol and an electronegativity of 2.2, high for a metal, explain why bulk osmium is so reluctant to react.
Safety and the environment
Solid osmium metal is inert and harmless to handle. The danger lies entirely with osmium tetroxide, which forms slowly on powdered metal and is highly toxic. Its vapour attacks the eyes, and even brief exposure can cause temporary blindness and lung damage. Laboratories keep it in sealed ampoules and open them only in fume cupboards.
Osmium is too rare, and used in such small quantities, to be a pollutant in its own right; the main footprint comes from mining the platinum and nickel ores it accompanies. Catalysts and old alloys are worth recycling because the metal is valuable.
Did you know?
- Osmium and iridium are so close in density, 22.59 and 22.56 grams per cubic centimetre, that for much of the twentieth century scientists disagreed about which was heavier.
- The lighting company Osram takes its name from osmium and wolfram, the German word for tungsten, the two metals used in early light bulb filaments.
- Osmium tetroxide is used to stain cell membranes so they show up clearly under an electron microscope.
- Osmium can reach an oxidation state of +8, sharing the record for the highest oxidation state found in stable compounds.
- Osmium-187, formed by the slow decay of rhenium-187, lets geologists date rocks and meteorites that are billions of years old.
- Smithson Tennant discovered both osmium and iridium in the same black residue in 1803.
More in the transition metals
Column-mates come first, since elements in one column have outer electrons arranged in a similar way and tend to behave alike; the rest are the nearest neighbours in the family.
See all 38 transition metals on one page, or browse all 118 elements.
Compare Osmium with another element
Each comparison has a side-by-side data table and a plain-English summary of the differences.
Frequently asked questions
Is osmium the heaviest element?
It is the densest natural element, meaning it packs the most mass into a given volume. It is not the heaviest atom, though. Elements such as uranium and the synthetic elements have far heavier individual atoms but their atoms are spaced further apart.
Is osmium toxic?
The solid metal is not toxic and can be handled safely. Osmium tetroxide, which forms when the powdered metal reacts with air, is highly toxic and damages the eyes and lungs. It is handled only in fume cupboards.
What does osmium smell like?
Pure osmium metal has no smell. The name refers to osmium tetroxide, which has a sharp, acrid odour a bit like chlorine or ozone. Smelling it is a warning sign of dangerous exposure.
Why is osmium so rare and expensive?
Osmium makes up only a tiny fraction of the Earth's crust and has no ores of its own. It is recovered in small amounts as a by-product of platinum and nickel refining, so world production is only a few hundred kilograms a year.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Osmium reference article. Spotted an error? Tell us.