76 Os Osmium 190.23
Transition metal d-block Period 6 Group 8

Osmium

Os · Element 76 · Transition metal

Print or save as PDF Embed on your site

The densest naturally occurring element: a litre of osmium weighs 22.6 kg.

State at 20°C Solid
Atomic mass 190.23 u
Electron configuration[Xe] 4f¹⁴ 5d⁶ 6s²
75Re All 118 77Ir

Structure

The osmium atom

Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.

Orbital cloud

Measured values

Property sheet

Every bar shows where osmium sits among all 118 elements for that property.

Physical

Density 22.59 g/cm³ 92%
Melting point 3306 K 97%
Boiling point 5285 K 96%
Specific heat 0.13 J/g·K
Thermal conductivity 87.6 W/m·K 78%

Atomic

Atomic radius 135 pm 25%
Covalent radius 144 pm
Van der Waals radius 216 pm

Electronic

Electronegativity 2.2 78%
Ionisation energy 814.3 kJ/mol 70%
Electron affinity 106.1 kJ/mol 71%

Occurrence

Abundance in crust 0.002 mg/kg 35%

Identity

SymbolOs
Atomic number76
Atomic mass190.23 u
CategoryTransition metal
Blockd
Crystal structurehexagonal close-packed
Oxidation states-2, 0, +1, +2, +3, +4, +5, +6, +7, +8
Discovered1803
Discovered byTennant & Wollaston

Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.

Same element, different neutrons

Isotopes of osmium

All osmium atoms have the same number of protons, 76, and differ only in their number of neutrons: those variants are its isotopes. The most common, Os-192, has 116 neutrons and makes up 40.78% of natural osmium.

Isotopes of osmium found in nature, with their share of the natural element
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 most common 116 40.78% Stable —

The classroom shortcut, rounded atomic mass minus atomic number, gives 114 for osmium, but the most common osmium atom has 116 neutrons: the atomic mass, 190.23, is an average over all natural isotopes.

Decay modes

2β⁺
double positron emission
α
alpha decay

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.

Size, to scale

How big is an osmium atom?

Radius 135 pm — that is 0.135 nm, so about 3.7 million of them side by side would span a millimetre.

Thermal range

Solid, liquid, gas — and when

Osmium is liquid over a 1979 K window, from 3306 K to 5285 K.

Where it sits

Position in the table

Osmium sits in period 6, group 8. Everything in group 8 shares the same outer-electron count, which is why they behave so similarly.

Other transition metals

21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 39Y 40Zr 41Nb 42Mo 43Tc 44Ru
All transition metals

The story

What osmium is, and how we found it

Osmium (Os) is element 76, a hard, brittle, bluish-white metal in group 8 and one of the six platinum group metals. With a density of 22.59 g/cm³ it is the densest naturally occurring element, slightly denser than iridium. It melts at 3,033 °C (3,306 K) and boils at 5,012 °C (5,285 K).

Atomic structure and isotopes

Osmium has 76 electrons in six shells, arranged 2, 8, 18, 32, 14, 2, with the configuration [Xe] 4f¹⁴ 5d⁶ 6s². It reaches the +8 oxidation state in osmium tetroxide (OsO₄), one of very few elements to do so.

Natural osmium has seven isotopes; osmium-192 is the most abundant, at about 41%. Part of the osmium-187 in rocks comes from the slow radioactive decay of rhenium-187, which makes the rhenium and osmium pair a clock for dating ores, meteorites and oil.

The discovery of osmium

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%.

Applications

What osmium is used for

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.

In everyday life

Osmium is one of the least used metals, and world production is very small. You are unlikely to meet it except in the hard tips of some high-quality fountain pens and in older record player styluses. Crystallized osmium is also sold as a collector's and investment metal.

Where it comes from

Natural occurrence

0.002 mg/kg of Earth's crust · more abundant than 35% of elements

Abundance

Osmium is one of the rarest elements in Earth's crust, at about 0.002 parts per million. Most of Earth's osmium is thought to have sunk into the core as the planet formed, and meteorites contain far more of it than crustal rocks.

Minerals

It occurs as natural alloys with iridium, called osmiridium and iridosmine, in platinum-bearing river gravels and in nickel and copper ores, and in rare minerals such as erlichmanite (osmium sulfide).

Production

Osmium is recovered in small quantities as a byproduct of platinum and nickel refining, mainly in South Africa, Russia and Canada.

Handling

Safety

Osmium tetroxide

Solid osmium metal is not very hazardous, but osmium powder slowly reacts with the oxygen in air, and heated osmium burns, to form osmium tetroxide (OsO₄). This volatile, highly toxic compound has a sharp, chlorine-like smell. Its vapor can damage the cornea and cause temporary or permanent blindness, and it irritates the lungs and skin.

Exposure limits

In the United States the OSHA limit for osmium tetroxide in workplace air is 0.002 mg per cubic meter, one of the lowest limits for any substance.

Safe handling

  • Handle osmium tetroxide only in a fume hood, in sealed ampoules or solutions, with chemical goggles and gloves.
  • Do not heat osmium powder in air or grind it outside an inert atmosphere.
  • Treat spills of osmium tetroxide with corn oil or sodium sulfite, which reduce it to less hazardous compounds.

Keep exploring

Use osmium in the tools

Quick answers

Osmium: common questions

What is osmium?

Osmium (symbol Os) is element 76 on the periodic table, a transition metal in period 6, group 8. The densest naturally occurring element: a litre of osmium weighs 22.6 kg. At room temperature it is a solid.

What is the electron configuration of osmium?

Osmium's ground-state electron configuration is [Xe] 4f¹⁴ 5d⁶ 6s², giving 6 occupied shells with the distribution 2, 8, 18, 32, 14, 2.

What are the melting and boiling points of osmium?

Osmium melts at 3306 K (3032.9 °C) and boils at 5285 K (5011.9 °C). That puts it among the most refractory elements — it stays solid at temperatures that vaporise most metals.

What is the atomic mass of osmium?

The standard atomic weight of osmium is 190.23 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.

How many neutrons does osmium have?

The most common osmium atom, Os-192, has 116 neutrons: its mass number, 192, minus its atomic number, 76. It makes up 40.78% of natural osmium. The classroom shortcut, rounded atomic mass (190.23) minus atomic number, gives 114 instead, because the atomic mass is an average over all natural isotopes of osmium.

How dense is osmium?

Osmium has a density of 22.59 g/cm³. Water is 1.0 g/cm³, so a block of osmium is about 22.6× heavier.

What is the electronegativity of osmium?

Osmium has a Pauling electronegativity of 2.2. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.

Who discovered osmium, and when?

Osmium was discovered in 1803 by Tennant & Wollaston. Origin of the name: Greek osme, "smell".

How common is osmium on Earth?

Osmium makes up about 0.002 mg/kg of the Earth's crust — genuinely rare, which is why it is expensive.

← Previous Re Rhenium Element 75 · Transition metal Next → Ir Iridium Element 77 · Transition metal