Osmium
Osmium (from Greek osme (ὀσμή) meaning "smell") is a chemical element with symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mostly in platinum ores. Osmium is the densest naturally occurring element, with a density of 22.59 g/cm3.
Identity
- Atomic mass
- 190.233 u
- Category
- Transition metals
- Group
- 8
- Period
- 6
- Block
- d
- Oxidation States
- +2, +3, +4, +6, +8
- Phase (25 °C)
- Solid
- Appearance
- silvery, blue cast
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d6
- Short configuration
- [Xe] 4f14 5d6 6s2
- Electrons per shell
- 2, 8, 18, 32, 14, 2
- Electronegativity (Pauling)
- 2.20
- Electron affinity
- 104 kJ/mol
- 1st ionization energy
- 840 kJ/mol
- Ionization energies
- 840 · 1600 kJ/mol
Physical properties
- Melting point
- 3032.8 °C (3306 K)
- Boiling point
- 5011.9 °C (5285 K)
- Density
- 22.59 g/cm³
- Molar heat capacity
- 24.7 J/(mol·K)
Chemical behavior of osmium
Osmium has the electron configuration [Xe] 4f¹⁴ 5d⁶ 6s². It is the densest natural chemical element in the periodic table (22.59 g/cm³) and an extremely hard, brittle, refractory platinum-group metal. It shows oxidation states ranging from −2 to +8, with the +4 state being the most stable in solution and the +8 state well known for its volatile tetroxide (OsO₄). It is a noble metal highly resistant to acid attack at ordinary temperature, but as a fine powder it slowly oxidizes in air releasing toxic osmium tetroxide vapors of penetrating, acrid odor.
Reactivity
Os + 2 O₂ → OsO₄
At moderate temperatures (or as a fine powder in air) it oxidizes to form osmium tetroxide, a very volatile, sublimable pale-yellow crystalline solid.
OsO₄ + 2 C=C + 2 H₂O → cis-1,2-diols + OsO₂
In organic chemistry it acts as a selective stereospecific oxidant for the syn dihydroxylation of alkenes (cis vicinal diols).
Os + 2 Cl₂ → OsCl₄
It reacts with chlorine gas above 650 °C to form dark-red osmium tetrachloride.
Os + 3 F₂ → OsF₆
It combines with fluorine gas at 300 °C forming gaseous yellow osmium hexafluoride.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Osmium tetroxide | OsO₄ | Selective oxidizing agent in organic synthesis (syn dihydroxylation) and fixative and stain for lipids in transmission electron microscopy |
| Osmium(III) chloride | OsCl₃ | Key precursor salt in the synthesis of organometallic coordination complexes and hydrogenation catalysts |
| Osmium hexafluoride | OsF₆ | Volatile valence-VI inorganic compound used in crystallographic research and advanced fluorination |
| Potassium osmate dihydrate | K₂[OsO₄(OH)₂] | Stable crystalline salt that serves as a practical, safe source of osmium for asymmetric catalysis reactions |
Electron shell distribution
View in the atom builderIsotopes
Natural osmium consists of seven isotopes: six stable ones (¹⁸⁴Os at 0.02%, ¹⁸⁷Os at 1.96%, ¹⁸⁸Os at 13.24%, ¹⁸⁹Os at 16.15%, ¹⁹⁰Os at 26.26% and ¹⁹²Os at 40.78%) and a primordial alpha-emitting radionuclide with a colossal half-life, osmium-186 (¹⁸⁶Os, 1.59%, half-life of 2.0 × 10¹⁵ years). The osmium-187 isotope is radiogenic, generated by the beta decay of rhenium-187; the ¹⁸⁷Os/¹⁸⁸Os ratio is an essential geochemical tracer for identifying extraterrestrial meteorite impacts and deep mantle magmatism.
Occurrence and production
It is one of the rarest metals in the Earth's crust, with an average concentration of only 0.05 parts per billion (0.05 ppb). It appears in the native state alloyed with iridium in the minerals osmiridium and iridosmium in river placers, and as an impurity in nickel and copper sulfides of large igneous complexes such as the Bushveld complex (South Africa) and Norilsk (Russia). Its extraction is complex: after dissolving the platinum-metal concentrate in aqua regia, osmium remains in the insoluble residue and is separated by oxidative distillation of its volatile tetroxide.
History and origin of the name
It was discovered in London in 1803 by the British chemist Smithson Tennant. While dissolving crude platinum in aqua regia, Tennant investigated the black insoluble residue that other chemists considered graphite, demonstrating that it contained two new metals: osmium and iridium. Tennant named it 'osmium' from the Greek term 'osmē' (ὀσμή, smell or stench), in allusion to the penetrating, suffocating, acrid odor given off by the volatile vapor of its freshly formed tetroxide.
Applications and relevance of osmium
Uses of the element
Thanks to its extreme hardness and resistance to mechanical wear and corrosion, osmium alloys with iridium and ruthenium have been used in the tips of high-end fountain pens, marine compass pivots, vinyl record player needles and electrical contacts in precision navigation instruments. It is also being researched in superconducting alloys and light-emitting diodes.
Uses of its compounds
Osmium tetroxide (OsO₄) is the universal, irreplaceable reagent for staining lipid cell membranes in transmission electron microscopy (TEM), providing great contrast to the electron beams. In pharmaceutical synthesis it is used in the Sharpless asymmetric dihydroxylation to manufacture chiral drugs of high optical purity.
Health, safety and environment
Biological role
It plays no biological role in living organisms. In bulk metallic form it is biologically inert and non-toxic, but its dust or any derivative that generates osmium tetroxide poses a formidable chemical hazard.
Safety
Osmium tetroxide is highly toxic, volatile and corrosive. Vapors at concentrations of just 10⁻⁷ g/L can oxidize and fix the lipids of the corneas, causing permanent blindness, in addition to severe pulmonary edema and irreversible dark skin burns. It must be handled in a fume hood with double gloves and reducing agents such as corn oil ready to deactivate it.
Environment
Due to its scarcity and economic value, it is recycled almost 100% in closed industrial circuits. Its emissions to the environment are practically nil and no bioaccumulation is known in aquatic ecosystems.
Frequently asked questions about Osmium
Why is osmium so heavy compared with other metals?
Osmium is the densest natural element that exists: an osmium soccer ball would weigh over a hundred kilograms. Its atoms have many protons that strongly attract electrons toward the nucleus, keeping them very compact. When bonded in an extremely tight crystal lattice, osmium packs enormous mass into a diminutive volume, surpassing even gold and lead.
How did the osmium isotopic signature demonstrate the impact that wiped out the dinosaurs?
Earth's continental crust rocks have a very high osmium-187 to osmium-188 ratio due to rhenium decay over billions of years. In contrast, extraterrestrial meteorites preserve very low primordial solar-system isotopic ratios. When analyzing the thin clay layer at the Cretaceous–Paleogene boundary worldwide, geochemicists found an osmium isotopic signature identical to that of chondritic asteroids, confirming the Chicxulub impact.
Why did classic fountain pens carry osmium in the nib?
Writing by hand on paper for years produces continuous friction that would quickly wear down soft metals such as gold or silver. To solve this, manufacturers placed a tiny ball of a natural osmium–iridium alloy at the tip of the nib. Its extreme hardness and resistance to corrosion by acidic inks allowed a lifetime of writing without losing smoothness.
Why is osmium brittle at room temperature despite being a transition metal?
Osmium crystallizes in a hexagonal close-packed (HCP) lattice that has a very limited number of independent atomic slip planes at ordinary temperature. In addition, its interatomic bonds combine metallic bonding with a marked directional covalent character due to the overlap of the half-filled 5d orbitals. When subjected to twisting or mechanical impact, the atomic planes cannot slide to absorb the plastic strain, causing the metal to fracture in a brittle manner.
Why is osmium tetroxide used to observe cells under electron microscopes?
Osmium tetroxide reacts with the fatty components of cell membranes and helps preserve their structure during sample preparation. In addition, osmium scatters electrons efficiently, so treated areas appear with greater contrast under the electron microscope. It is a highly toxic and volatile substance, so it should only be handled in laboratories equipped for it.
What is the Nobel-winning Sharpless asymmetric dihydroxylation?
In pharmaceutical synthesis, turning an alkene double bond into two neighboring alcohol groups oriented in a single three-dimensional direction is a major challenge. Karl Barry Sharpless developed a catalytic system in which a tiny catalytic amount of osmium tetroxide, combined with a chiral ligand derived from quinine, introduces two hydroxyl groups onto a specific face of the molecule with nearly 100% optical purity, a discovery recognized with the Nobel Prize in Chemistry in 2001.
What should be done in case of an osmium tetroxide spill?
Osmium tetroxide is highly toxic and can release dangerous vapors. A spill should not be treated like a household product nor cleaned up improvising with oil or other substances. Leave the area, avoid inhaling the vapors and alert safety personnel or a specialized team. Handling it requires protective equipment and specific procedures.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.