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Osmium,  76Os
General properties
Name, symbol Osmium, Os
Appearance Silvery, bluish cast
Osmium in the periodic table


Atomic number 76
Standard atomic weight (Ar) 190.23(3)
Group, block , d-block
Period period 6
Electron configuration [Xe] 4f14 5d6 6s2
per shell
2, 8, 18, 32, 14, 2
Physical properties
Silvery with bluish cast
Phase Solid
Melting point 3306 K ​(3033 °C, ​​5491 °F)
Boiling point 5285 K ​(5012 °C, ​9054 °F)
Density near r.t. 22.59 g/cm3
when liquid, at  20 g/cm3
Heat of fusion 31 kJ/mol
Heat of 378 kJ/mol
Molar heat capacity 24.7 J/(mol·K)
Atomic properties
Oxidation states 8, 7, 6, 5, 4, 3, 2, 1, 0, −1, −2, −4 ​(a mildly acidic oxide)
Electronegativity Pauling scale: 2.2
energies 1st: 840 kJ/mol
2nd: 1600 kJ/mol
Atomic radius empirical: 135 pm
Covalent radius 144±4 pm
Crystal structure ​Hexagonal close-packed (hcp)
Speed of sound thin rod 4940 m/s (at 20 °C)
Thermal expansion 5.1 µm/(m·K) (at 25 °C)
Thermal conductivity 87.6 W/(m·K)
Electrical resistivity 81.2·10-9 Ω·m (at 0 °C)
Magnetic ordering Paramagnetic
Shear modulus 222 GPa
Bulk modulus 462 GPa
Poisson ratio 0.25
Mohs hardness 7.0
Brinell hardness 3490–4000 MPa
CAS Registry Number 7440-04-2
Discovery and first isolation Smithson Tennant (1803)
· references

Osmium is the chemical element with symbol Os and atomic number 76.


Chemical properties

Osmium forms compounds with oxidation state from -2 to 8. It is slightly attacked by aqua regia, and by fuming nitric acid, and it is attacked by molten hydroxides. It forms the toxic osmium tetroxide (OsO4), the most common compound with +8 oxidation state, in contact with air, though bulk metal is much less prone to forming the oxide than the powder. A lower oxide, osmium dioxide, is known. It forms coordination complexes easily.

Physical properties

Osmium is a hard, brittle, white-bluish transition metal and usually is found in traces in platinum ores. It is the densest non-radioactive element known, with a density of 22.6 g/cm3 (22.59 g/cm3 according to most sources), slightly denser than iridium. Osmium is almost twice as dense as lead. It has a very high melting point of 3033 ºC, which makes it unfeasible for the amateur chemist to cast.


Osmium is one of the rarest elements so it is quite expensive. It can be bought online from Metallium and sometimes from eBay.




Osmium metal poses very little toxicity as it does not react with most reagents. Osmium powder, however, will slowly oxidize in air to form osmium tetroxide, which is toxic and a powerful oxidant. Osmium powder is also known to be pyrophoric. The bulk metal, however, is safe to keep in air.


Bulk osmium does not require any special storage. Osmium powder should be kept in closed containers, in a well ventilated area, preferable under a gas other than air (such as carbon dioxide, nitrogen, argon, etc.).


Due to osmium's high price, it's best to try and recycle it as much as possible.


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