Osmium (Os)

Osmium (symbol Os) is the chemical element with atomic number 76: a transition metal, solid at 0 °C, with an atomic mass of 190.23 u. Discovered in 1803 (Smithson Tennant).

Osmium is the densest of all elements: a liter of it would weigh more than 22 kg. Its name comes from the Greek osme, “smell,” because of the acrid odor of its oxide, a highly toxic compound. It was once used for fountain-pen nibs and early light-bulb filaments.

Key facts

Atomic number: 76
Symbol: Os
French name: Osmium
Atomic mass: 190.23 u
Molar mass: 190.23 g/mol
Family: Transition metals
Position: period 6, group 8, d-block
State at 0 °C: Solid
Melting point: 3,032.9 °C
Boiling point: 5,011.9 °C
Density: 22.59 g/cm³
Electronegativity (Pauling): 2.2
First ionization energy: 814 kJ/mol
Atomic radius: 130 pm
Electron configuration: [Xe] 4f¹⁴ 5d⁶ 6s²
Abundance in Earth's crust: 0.0015 ppm (76th most abundant element)
Indicative price: 30,000 $/kg (order of magnitude)
Radioactive: no
Discovery: 1803, Smithson Tennant

Origin of the name

From the Greek osmē, smell, for the odor of its oxide.

What does osmium look like?

A shiny blue-gray metal, extremely hard and brittle. Its density reaches 22.59 g/cm³.

Where is osmium found?

One of the rarest elements in the crust (about 1.5 parts per billion). It occurs alongside platinum, notably as natural alloys with iridium.

Natural sources

Osmiridium
Iridosmine
Native platinum

How is osmium produced?

A by-product of nickel and platinum refining. Once the other metals are dissolved, osmium is oxidized to its volatile tetroxide, distilled, then reduced. Only a few hundred kilos are produced each year.

Top producers

South Africa
Russia
Canada

What is osmium used for?

Ultra-hard alloys for fountain-pen nibs, electrical contacts and pivots.
Osmium tetroxide, a stain for electron microscopy.
Fingerprint developer.
Catalysts in organic chemistry.

In everyday life

A high-end fountain-pen nib (formerly)

Role in living things

No biological role.

Hazards of osmium

The solid metal is harmless, but the powder slowly oxidizes in air to osmium tetroxide, which is highly toxic: it can cause pulmonary edema and serious eye damage, even blindness.

Cosmic origin

Neutron stars: Created when two neutron stars collide — the “forge” of gold.

History of osmium

In 1803 the Englishman Smithson Tennant was studying the black residue left when platinum is dissolved in aqua regia. In it he found two elements: iridium and osmium, the latter named for its smell.

Did you know?

Osmium and iridium are so close in density that it took very precise X-ray measurements to crown the champion.
The OSRAM brand takes its name from osmium, used in Auer von Welsbach's light-bulb filaments around 1900.
A cube of osmium 10 cm on a side would weigh nearly 23 kg.

Isotopes of osmium

Osmium has seven natural isotopes, osmium-192 being the most abundant. The share of osmium-187, fed by radioactive rhenium, tells the geological story of rocks.

osmium-184: 0.02% of natural osmium, stable
osmium-186: 1.59% of natural osmium, radioactive, half-life 2 × 10¹⁵ years
osmium-187: 1.96% of natural osmium, stable. Produced by rhenium-187: the key to rhenium–osmium dating.
osmium-188: 13.24% of natural osmium, stable
osmium-189: 16.15% of natural osmium, stable
osmium-190: 26.26% of natural osmium, stable
osmium-191: radioactive, half-life 15.4 days
osmium-192: 40.78% of natural osmium, stable
osmium-194: radioactive, half-life 6 years

Related elements

Same family (transition metals): Scandium (Sc), Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Yttrium (Y), Zirconium (Zr), Niobium (Nb), Molybdenum (Mo), Technetium (Tc), Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Silver (Ag), Cadmium (Cd), Hafnium (Hf), Tantalum (Ta), Tungsten (W), Rhenium (Re), Iridium (Ir), Platinum (Pt), Gold (Au), Mercury (Hg), Rutherfordium (Rf), Dubnium (Db), Seaborgium (Sg), Bohrium (Bh), Hassium (Hs).

Same column (group 8): Iron (Fe), Ruthenium (Ru), Hassium (Hs).