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Chemical element, symbol Os and atomic number 76 / From Wikipedia, the free encyclopedia

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Osmium (from Ancient Greek ὀσμή (osmḗ) 'smell') is a chemical element; it has symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mostly in platinum ores. Osmium is the densest naturally occurring element. When experimentally measured using X-ray crystallography, it has a density of 22.59 g/cm3.[6] Manufacturers use its alloys with platinum, iridium, and other platinum-group metals to make fountain pen nib tipping, electrical contacts, and in other applications that require extreme durability and hardness.[7]

Quick facts: Osmium, Pronunciation, Appearance, Standard a...
Osmium, 76Os
Pronunciation/ˈɒzmiəm/ (OZ-mee-əm)
Appearancesilvery, blue cast
Standard atomic weight Ar°(Os)
Osmium in the periodic table


Atomic number (Z)76
Groupgroup 8
Periodperiod 6
Block  d-block
Electron configuration[Xe] 4f14 5d6 6s2
Electrons per shell2, 8, 18, 32, 14, 2
Physical properties
Phase at STPsolid
Melting point3306 K (3033 °C, 5491 °F)
Boiling point5285 K (5012 °C, 9054 °F)
Density (near r.t.)22.59 g/cm3
when liquid (at m.p.)20 g/cm3
Heat of fusion31 kJ/mol
Heat of vaporization378 kJ/mol
Molar heat capacity24.7 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 3160 3423 3751 4148 4638 5256
Atomic properties
Oxidation states−4, −2, −1, 0, +1, +2, +3, +4, +5, +6, +7, +8 (a mildly acidic oxide)
ElectronegativityPauling scale: 2.2
Ionization energies
  • 1st: 840 kJ/mol
  • 2nd: 1600 kJ/mol
Atomic radiusempirical: 135 pm
Covalent radius144±4 pm
Color lines in a spectral range
Spectral lines of osmium
Other properties
Natural occurrenceprimordial
Crystal structure hexagonal close-packed (hcp)
Hexagonal close packed crystal structure for osmium
Thermal expansion5.1 µm/(m⋅K) (at 25 °C)
Thermal conductivity87.6 W/(m⋅K)
Electrical resistivity81.2 nΩ⋅m (at 0 °C)
Magnetic orderingparamagnetic[3]
Molar magnetic susceptibility11×10−6 cm3/mol[3]
Shear modulus222 GPa
Bulk modulus462 GPa
Speed of sound thin rod4940 m/s (at 20 °C)
Poisson ratio0.25
Mohs hardness7.0
Vickers hardness4137 MPa
Brinell hardness3920 MPa
CAS Number7440-04-2
Discovery and first isolationSmithson Tennant (1803)
Isotopes of osmium
Main isotopes[4] Decay
abun­dance half-life (t1/2) mode pro­duct
184Os 0.02% 1.12×1013 y[5] α 180W
185Os synth 92.95 d ε 185Re
186Os 1.59% 2.0×1015 y α 182W
187Os 1.96% stable
188Os 13.2% stable
189Os 16.1% stable
190Os 26.3% stable
191Os synth 14.99 d β 191Ir
192Os 40.8% stable
193Os synth 29.83 h β 193Ir
194Os synth 6 y β 194Ir
Symbol_category_class.svg Category: Osmium
| references

Osmium is among the rarest elements in the Earth's crust, making up only 50 parts per trillion (ppt).[8][9] It is estimated to be about 0.6 parts per billion in the universe and is therefore the rarest precious metal.[10]

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