Osmium
Atomic Data
| Atomic Number | 76 |
| Symbol | Os |
| Atomic Weight | 190.23 u |
| Density (STP) | 22.59 g/cm³ |
| Melting Point | 3032.85 °C (3306 K) |
| Boiling Point | 5011.85 °C (5285 K) |
| Electronegativity | 2.2 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d6 6s2 |
| Oxidation States | +4, +8 |
| Phase at STP | Solid |
| Category | Transition Metal |
| Period / Group | 6 / 8 |
| CAS Number | 7440-04-2 |
Electron Configuration
[Xe] 4f14 5d6 6s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 6 | 74 |
| P | 6 | 6s | 2 | 76 |
| Total | 76 | 76 | ||
Isotopes of Osmium
Osmium has seven naturally occurring stable isotopes. The most abundant is ¹⁹²Os, comprising 40.78% of all naturally occurring Osmium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Osmium-184 | ¹⁸⁴Os | 76 | 108 | 0.02 | Stable |
| Osmium-186 | ¹⁸⁶Os | 76 | 110 | 1.59 | Stable |
| Osmium-187 | ¹⁸⁷Os | 76 | 111 | 1.96 | Stable |
| Osmium-188 | ¹⁸⁸Os | 76 | 112 | 13.24 | Stable |
| Osmium-189 | ¹⁸⁹Os | 76 | 113 | 16.15 | Stable |
| Osmium-190 | ¹⁹⁰Os | 76 | 114 | 26.26 | Stable |
| Osmium-192 | ¹⁹²Os | 76 | 116 | 40.78 | Stable |
Abundance & Occurrence
Osmium is present in Earth's crust at approximately 0.0018 ppm by mass and at approximately 0.3 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Osmium tetroxide (OsO4): severe hazard: OsO4 is a highly volatile, acutely toxic solid that severely damages eyes (causing temporary blindness and corneal scarring) and the respiratory tract at very low vapour concentrations; any oxidising treatment of osmium must be performed in a well-ventilated fume hood.
- Inhalation and skin exposure: Even brief exposure to OsO4 vapour causes a characteristic black discolouration of mucous membranes and conjunctivae; pulmonary oedema can follow significant inhalation exposure.
- Formation from bulk osmium: Osmium metal exposed to air slowly forms OsO4; bulk osmium should be stored in sealed containers, and oxidative reactions must be conducted with extreme care.
- Osmium powder fire hazard: Fine osmium powder is flammable; metal fires require Class D extinguishing agents.
Osmium in the Real World
Real-World Uses
- Wear-resistant alloy tips: Osmium-iridium alloy (and pure osmium) was historically used for the tips of fountain pen nibs and gramophone stylii because of its extreme hardness and wear resistance; iridium-based alloys have largely replaced osmium in modern pen nibs.
- Biological tissue staining: Osmium tetroxide (OsO₄) reacts with unsaturated lipids in biological membranes to fix and stain specimens for transmission electron microscopy (TEM), providing high-contrast delineation of membrane-bound organelles and lipid bilayers.
- Fingerprint detection: Osmium tetroxide vapour reacts with fatty acids in latent fingerprints to form a dark osmium deposit, used by forensic scientists to develop prints on paper and non-porous surfaces resistant to conventional methods.
- Asymmetric dihydroxylation catalyst: Osmium tetroxide is the reagent in the Sharpless asymmetric dihydroxylation, which converts alkene double bonds to vicinal diols with high enantioselectivity; it is an important tool in the synthesis of pharmaceutical active ingredients.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is osmium used for?
Osmium is used in specialised alloys requiring extreme hardness and wear resistance: osmium-iridium alloys were historically used for fountain pen nibs and compass needles. Osmium tetroxide (OsO4) is used as a staining agent in electron microscopy, where it reacts with carbon-carbon double bonds in lipids, providing contrast for biological specimens. It is also used as a catalyst in organic synthesis.
Is osmium the densest element?
Osmium and iridium are the two densest elements, and they compete closely for the record. Modern measurements give osmium a density of 22.59 g/cm3 and iridium 22.56 g/cm3, making osmium the densest natural element. An osmium sphere the size of a baseball would weigh about 10 kg. For comparison, lead is only about 11.3 g/cm3.
How was osmium discovered?
Osmium was discovered in 1803 by British chemist Smithson Tennant while working with the residue left after platinum was dissolved in aqua regia. The same residue also yielded iridium. Tennant named osmium after the Greek 'osme', meaning smell, because osmium tetroxide: formed when osmium reacts with oxygen: has a distinctively pungent, acrid odour.
Why is osmium tetroxide so dangerous?
Osmium tetroxide (OsO4) is a potent oxidant and highly toxic. Even at very low vapour concentrations it severely damages eyes and mucous membranes, and can cause blindness. It is absorbed through the skin and converted in tissue to osmium dioxide, which stains flesh dark and causes chemical burns. Despite its extreme hazard, OsO4 is irreplaceable in transmission electron microscopy for fixing and staining lipid-rich biological membranes, so it continues to be used with strict safety precautions.