Atomic number 76

Osmium

Os
+ phase
− phase

Shells Orbitals 4f 5d 6s

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76 electrons in 6 shells: 2, 8, 18, 32, 14, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Transition metal
Period: 6
Group: 8
Block: d
Standard atomic weight: 190.23(3) Da
Electron configuration: [Xe] 4f¹⁴ 5d⁶ 6s²
Phase at 25 °C: solid
Melting point: 3033 °C
Boiling point: 5012 °C
Density: 22.57 g/cm³
Electronegativity (Pauling): 2.2
Discovered: 1803, Smithson Tennant

Neighbours in the table

Re Ir

Los Alamos reports that the two ways of finding the densities of osmium and iridium disagree. PubChem’s record gives osmium 22.57 g/cm³, while measured and lattice-calculated figures for the pair point in opposite directions.

In osmium the configuration [Xe] 4f¹⁴ 5d⁶ 6s² corresponds to shells holding 2, 8, 18, 32, 14 and 2. That full fourth shell of 32 is the work of the lanthanides, OpenStax explains: from lanthanum to lutetium, fourteen f electrons bring it up from 18. For a transition metal the textbook counts the outer s and inner d electrons as valence electrons, eight for osmium. When the atom forms ions, the 6s pair leaves first.

Los Alamos lists oxidation states from +8 all the way down to −4, each a bookkeeping charge that treats every bond as ionic. Jefferson Lab, in PubChem’s record, gives only +4 and +3, the common ones. OpenStax notes that heavier transition metals hold high states more readily: their ions are larger, and electrons far from the nucleus come away more easily. Los Alamos describes the metal as a lustrous bluish white, extremely hard, and still brittle when hot.

Where it is found

Smithson Tennant found osmium in 1803 in a black residue. Crude platinum leaves it behind after dissolving in aqua regia, a mixture of nitric and hydrochloric acids. Iridium turned up in the same residue. IUPAC’s atomic-weights commission (CIAAW) adds that the French chemist Collet-Descotils found iridium independently; Tennant got the credit, perhaps because he alone also found osmium.

Jefferson Lab puts osmium in the Earth’s crust at 1.5 × 10⁻³ milligrams per kilogram. Los Alamos places it in platinum-bearing river sands of the Urals and of North and South America, and in the nickel ores around Sudbury, Ontario. The platinum metals in those ores are sparse, but so many tonnes of nickel ore are processed that recovering them pays. Most osmium today comes out of platinum and nickel ore processing.

The US Geological Survey records American imports of osmium at one kilogram in each year from 2020 to 2024 except 2023, when none was recorded. The 2024 figure is an estimate.

Why it matters

Jefferson Lab ties osmium’s uses to how awkward the metal is to handle. Powder is easier to make than solid pieces, but it gives off osmium tetroxide, OsO₄, in air. So the element goes mainly into very hard alloys with the other platinum metals. Los Alamos lists pivots for instruments, needles for phonographs, electrical contacts and the tips of fountain pens, all parts where wear from friction has to stay small. The tetroxide itself has been used to bring out fingerprints and to stain fatty tissue on microscope slides, Los Alamos adds.

The isotopes carry a clock. CIAAW lists seven in natural osmium, from osmium-184 to osmium-192, the last making up about 41 per cent; Jefferson Lab’s entry counts five of them as stable. Some osmium-187 comes from the decay of radioactive rhenium-187, whose half-life is 4.16 × 10¹⁰ years. Variations in these isotope ratios have been used to date the Earth, the Moon and meteorites, IUPAC’s isotope notes say.

Meteorites and their dust carry osmium of a different isotopic make-up from terrestrial rock. As a result, osmium-187 to osmium-186 ratios point to extraterrestrial material still arriving over geological time. Osmium-192 also serves as the starting point for platinum-195m, a medical radioisotope, IUPAC’s isotope notes say.

Properties

Properties of osmium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	76
Standard atomic weight	190.23(3) Da	Da	IUPAC/CIAAW 2024
Melting point	3033 °C	°C	Los Alamos: 3033 °C
Boiling point	5012 °C	°C	Los Alamos: 5012 °C
Density	22.57 g/cm³	g/cm³
Electronegativity (Pauling)	2.2
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁶
Phase at 25 °C	solid

Safety

Solid osmium is inert, PubChem’s safety record says, but the powder is not. Company notifications under the Globally Harmonized System (GHS) class elemental osmium as a flammable solid, signal word Danger, that can seriously damage the eyes and irritate skin and airways. Finely divided metal, or metal heated in air, oxidises to osmium tetroxide, which Los Alamos calls highly toxic. The PubChem Laboratory Chemical Safety Summary for osmium holds the detail for anyone handling it.

Questions

Is osmium the densest element?

By measurement, yes: measured values make osmium a little denser than iridium, Los Alamos reports, and PubChem's record gives osmium 22.57 g/cm³. Density worked out from the spacing of atoms in each crystal tips the other way, giving iridium 22.65 g/cm³ and osmium 22.61 g/cm³.

Why is osmium called osmium?

The name is Greek: osme means smell. IUPAC's atomic-weights commission, quoted in PubChem's record, says it was chosen for the sharp odour of the metal's volatile oxide. That oxide, osmium tetroxide, comes off the metal itself: Los Alamos notes that powdered or spongy osmium slowly gives it off, with a strong smell. Smithson Tennant found the element in 1803 in crude platinum ore.

What is osmium used for?

Mostly in very hard alloys. Jefferson Lab, in PubChem's record, names ballpoint and fountain pen tips, record player needles and electrical contacts, anywhere frictional wear must be kept down. Iridium–osmium alloy also tips fountain pens and turns up in compass bearings, the iridium record adds. One isotope, osmium-192, is a starting material for platinum-195m, a medical radioisotope, IUPAC's isotope notes say.

Is osmium dangerous?

Solid metal is inert, PubChem's safety record says, but finely divided osmium, or metal heated in air, turns into osmium tetroxide, which Los Alamos calls highly toxic. The same record says osmium dust is flammable when exposed to heat or flame, and names the tetroxide vapour as the hazard for people who work with the metal. PubChem's Laboratory Chemical Safety Summary for osmium gives the full classification.