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Name, Symbol, NumberOsmium, Os, 76
Chemical series transition metals
Group, Period, Block8 (VIIIB), 6 , d
Density, Hardness 22610 kg/m3, 7
Appearance bluish grey
Atomic Properties
Atomic weight 190.23 amu
Atomic radius (calc.) 130 (185) pm
Covalent radius 128 pm
van der Waals radius no data
Electron configuration [Xe]44f145d66s2
e- 's per energy level2, 8, 18, 32, 14, 2
Oxidation states (Oxide) 0.7 (mildly acidic)
Crystal structure hexagonal
Physical Properties
State of matter Solid (__)
Melting point 3306 K (5491 F)
Boiling point 5285 K (9054 F)
Molar volume 8.42 ×1010-3 m3/mol
Heat of vaporization 627.6 kJ/mol
Heat of fusion 31.8 kJ/mol
Vapor pressure 2.52 Pa at 3300 K
Speed of sound 4940 m/s at 293.15 K
Electronegativity 2.2 (Pauling scale)
Specific heat capacity 130 J/(kg*K)
Electrical conductivity 10.9 106/m ohm
Thermal conductivity 87.6 W/(m*K)
1st ionization potential 840 kJ/mol
2nd ionization potential 1600 kJ/mol
Most Stable Isotopes
isoNAhalf-life DMDE MeVDP
184Os0.02%>5.6 E13 y    
186Os1.58%>2.0 E15 y &alpha2.822182W
187Os1.6%Os is stable with 111 neutrons
188Os13.3%Os is stable with 112 neutrons
189Os16.1%Os is stable with 113 neutrons
190Os24.6%Os is stable with 114 neutrons
192Os41.0%Os is stable with 116 neutrons
194Os{syn.}6 y β-0.097194Ir
SI units & STP are used except where noted.
Osmium is a chemical element in the periodic table that has the symbol Os and atomic number 76. A hard brittle blue-gray or blue-black transition metal in the platinum family, osmium is one of, if not the heaviest metal known and is used in some alloys with platinum and iridium. Osmium is found native as an alloy in platinum ore and its tetroxide has been used to stain tissues and in fingerprinting. Alloys of osmium are employed in ballpoint pen tips, electrical contacts and in other applications where extreme durability and hardness are needed.

Notable Characteristics

Osmium in a metallic form is extremely dense, blue white, brittle and lustrous even at high temperatures, but proves to be extremely difficult to make. Powdered osmium is easier to make, but powdered osmium exposed to air leads to the formation of osmium tetroxide (OsO4), which is toxic. The oxide is also a powerful oxidizing agent emits a strong smell and boils at 130°C.

Due to its very high density osmium is generally considered to be the heaviest known element narrowly defeating iridium. However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22650 for iridium versus 22661 for osmium. Definitive selection between the two is therefore not possible at this time. It's just too close to call.

This metal has the highest melting point and the lowest vapor pressure of the platinum family. Common oxidation states of osmium are +4 and +3, but oxidation states from +1 to +8 are observed.


Because of the extreme toxicity of its oxide, osmium is rarely used in its pure state, and is instead often alloyed with other metals that are used in high wear applications. Osmium alloys are very hard and along with other platinum group metals is almost entirely used in alloys employed in the tips of ballpoint pens, phonograph needles, instrument pivots, and electrical contacts.

Osmium tetroxide has been used in fingerprint detection and in staining fatty tissue for microscope slides. An alloy of 90% platinum and 10% osmium (90/10) is used in surgical implants such as pacemakerss and replacement pulmonary valves.


Osmium (Greek osme meaning "a smell") was discovered in 1803 by Smithson Tennant in London, England along with iridium in the residue of dissolving platinum in aqua regia.


This transition metal is found in iridiosmium a naturally occurring alloy of iridium and osmium and in
platinum-bearing river sands in the Ural Mountains, North, and South America. It is also occurs in nickel-bearing ores found in the Sudbury, Ontario region with other platinum group metals. Even though the quantity of platinum metals found in these ores is small, the large volume of nickel ores processed makes commercial recovery possible.


Osmium tetroxide OsO4


Osmium has seven naturally-occurring
isotopes, 5 of which are stable: Os-187, Os-188, Os-189, Os-190, and (most abundant) Os-192. Os-184, Os-186 have absurdly long half lifes and for practical purposes can be consided to be stable as well. Os-187 is the daughter of rhenium-187 (half-life 4.56 x 1010 years) and is most often measured in a Os-187/Os-186 ratio. This ratio, as well as the Re-187/Os-187 ratio, have been used extensively in dating terrestrial as well as meteoric rockss. However, the most notable application of Os in dating has been in conjunction with iridium, to analyze the layer of shocked quartz along the K-T boundary that marks the extinction of the Dinosaurs 65 million years ago.


Osmium tetraoxide is highly
toxic. Airborne concentrations of osmium as low as 10-7 g/m can cause lung congestion, skin or eye damage.

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