Category: Transition metal
Atomic Number: 76
Period: 6 | Group: 8
Block: d-block
The densest naturally occurring element: a litre of osmium weighs 22.6 kg.
| Property | Value |
|---|---|
| Phase at STP | Solid |
| Density | N/A |
| Melting Point | 3306.00 K (3032.85 °C) |
| Boiling Point | 5285.00 K (5011.85 °C) |
| Heat Capacity | N/A |
| Property | Value |
|---|---|
| Atomic Mass | 190.2300 u |
| Electronegativity (Pauling) | N/A |
| Oxidation States | -2, 0, +1, +2, +3, +4, +5, +6, +7, +8 |
| Electron Configuration |
| Discovered By | Unknown |
|---|---|
| Discovery Year | 1803 |
| Named After | Greek osme, "smell" |
The black residue (1803)
In 1803 the English chemist Smithson Tennant, who worked with William Hyde Wollaston on refining platinum, dissolved crude platinum in aqua regia and studied the black insoluble residue left behind, which others had taken to be graphite. He found two new metals in it, osmium and iridium. French chemists working on the same residue reached similar results the same year.
The name
Tennant named osmium from the Greek osme, smell, because of the sharp, pungent odor of the volatile oxide formed when the residue was heated with alkali.
The densest element
For a long time it was unclear whether osmium or iridium was the denser metal. Precise X-ray measurements of their crystal structures settled the question in favor of osmium, by about 0.1%.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Os-184 | 108 | 0.02% | >5.6 × 10¹³ yrs | 2β⁺ |
| Os-186 | 110 | 1.59% | 2.0 × 10¹⁵ yrs | α |
| Os-187 | 111 | 1.96% | Stable | - |
| Os-188 | 112 | 13.24% | Stable | - |
| Os-189 | 113 | 16.15% | Stable | - |
| Os-190 | 114 | 26.26% | Stable | - |
| Os-192 | 116 | 40.78% | Stable | - |
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Hard alloys
Osmium is too brittle, and too hazardous when heated in air, to be used on its own. Alloys of osmium with iridium, platinum or other platinum group metals are very hard and wear-resistant, and have been used for fountain pen nib tips, phonograph needles, instrument pivots and electrical contacts.
Chemistry and microscopy
Electron microscopy: osmium tetroxide fixes and stains fats in biological samples, making cell membranes visible.
Organic synthesis: osmium tetroxide adds two hydroxyl groups across carbon-carbon double bonds; the asymmetric version developed by K. Barry Sharpless was recognized by the 2001 Nobel Prize in Chemistry.
Geology
Rhenium-osmium dating measures the age of sulfide ores, petroleum source rocks and meteorites, and osmium isotopes in ocean sediments record past changes in weathering and large meteorite impacts, including the one that ended the Cretaceous period.
| Earth's Crust | N/A |
|---|---|
| Universe | N/A |