Osmium

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. 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.

This article is about the chemical element. For other uses, see Osmium (disambiguation).

Osmium, 76Os
Osmium
Pronunciation/ˈɒzmiəm/ ​(OZ-mee-əm)
Appearancesilvery, blue cast
Standard atomic weight Ar°(Os)
Osmium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
 Ru
↑
Os
↓
Hs
rhenium ← osmium → iridium
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
Spectral lines of osmium
Other properties
Natural occurrenceprimordial
Crystal structure ​hexagonal close-packed (hcp)
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
Molar magnetic susceptibility11×10−6 cm3/mol
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
History
Discovery and first isolationSmithson Tennant (1803)
Isotopes of osmium
Main isotopes Decay
abun­dance half-life (t1/2) mode pro­duct
184Os 0.02% 1.12×1013 y 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

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

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