Osmium (Os)

Osmium is a chemical element with the atomic number 76 in the periodic table. It’s one of the rarest chemical elements that occur in Earth’s crust, with 1.5×10-3 ppm of natural abundance. 

Being a member of the platinum group metals (PGM), this noble metal with a pungent odor and high toxicity has many applications in medicine and various industries. As a part of the platinum metals group, osmium is often alloyed with the other precious metals. In this way, osmium alloys are used for an added durability, firmness, and anti-corrosive protection of manufactured items. 

Chemical and Physical Properties of Osmium

Atomic number76
Atomic weight190.23 g.mol-1
GroupTransition Metal
Period6 (d-block)
ColorA bluish-white color with a silvery luster
Physical stateSolid at room temperature
Half-lifeFrom 0.64(6) milliseconds to 2.0(11)×1015 years
Density22.40 g.cm-3 at 20°C
Melting point3033°C, 5491°F, 3306 K
Boiling point5008°C, 9046°F, 5281 K
Ionic radius0.067 nm (+4)
Electronic shell[Xe] 4f14 5d6 6s2
UsesUsed to tip gold pen points, instrument pivots, electric light
filaments, high temp. alloys, pressure bearings. Very hard
and corrosion-resistant.
DescriptionHeavy, brittle, white metal.
Crystal StructureHexagonal
Covalent Radius1.26 Å
Atomic Radius1.92 Å
Atomic Volume08.49 cm³/mol
Name OriginGreek: osmê (odor).
DiscovererSmithson Tenant
Oxydation States2,3,(4),6,8

With the periodic table symbol Os, atomic number 76, atomic mass of 190.2 g.mol-1, and electron configuration [Xe] 4f14 5d- 6s2, osmium is a hard and brittle bluish-white metal with a silvery lustre and pungent odor.

Osmium reaches its boiling point at 5008°C, 9046°F, 5281 K, while the melting point is achieved at 3033°C, 5491°F, 3306 K. In this regard, this chemical element has one of the highest melting and boiling points. 

This member of the platinum metals family of elements has an electronegativity of 2.2 according to Pauling, whereas the atomic radius according to van der Waals is 0.136 nm. Osmium metal has an excellent anti-corrosive property, and is resistant to wear and tear. Even after exposure to high temperatures, osmium does not lose its silvery luster.

How Was Osmium Discovered?

Osmium was discovered by the English chemists Smithson Tennant and William Hyde Wollaston in 1803. At that time, some scientists believed that crude platinum powder was actually graphite (C), archaically known as plumbago. However, Tennant’s and Wollaston’s curiosity spurred them to conduct additional experiments to obtain more valid evidence on this claim. 

These two discoverers then decided to implement the method for the isolation of palladium and rhodium out of platinum ore that had been conceptualized by Wollaston. Namely, Tennant and Wollaston tried to dissolve a sample of crude platinum in aqua regia (a mixture of hydrochloric acid and nitric acid) in a London laboratory. 

This experiment resulted in a metallic black powder. But, the English chemists were still convinced that there’s something more to this graphite-resembling substance. So, they attempted mixing the powder with sodium hydroxide after which they heated the mixture. By adding water to the residue, the alkali was removed and mixed with hydrochloric acid. 

This step of the experiment proved to be revolutionary because Tennant and Wollaston discovered an unknown metal element exactly in this solution. From the new element referred to as ‘ptene’, they succeeded in isolating a substance with an extremely strong smell – osmium tetroxide (OsO4). 

How Did Osmium Get Its Name?

As a result of its exceptionally strong and unpleasant smell, element 76 was given the name ‘osmium’. The term is derived from the Greek word  ‘osme‘, which means ‘smelly’.

Where Can You Find Osmium?

Osmium is the rarest and least abundant transition metal that naturally occurs in Earth’s crust. It’s mainly traced in alloys together with other platinum metals. Furthermore, element 76 is also traced in the mineral osmiridium (also known as iridosmine – an alloy of osmium and iridium). Osmiridium deposits are most often found in the gold-bearing conglomerates located in the Ural Mountains (Russia), Witwatersrand (South Africa), and California and Oregon (United States). 

This toxic chemical element can also be found in the minerals siserskite, aurosmiridium, as well as in native platinum ores. For commercial purposes, osmium is mostly obtained as a byproduct of nickel refining, i.e. from the processing of platinum and nickel ores.

Osmium in Everyday Life

As a member of the platinum metals group, the main application of this transition metal is in the making of very hard alloys that characterize with great resistance to wear. Osmium can be also applied in the following instances:

  • In medicine, osmium is used for making surgical implants, such as pacemakers and replacement heart valves;
  • Due to its toughness and resistance to wear, osmium is used in the manufacturing of fountain pen tips and phonograph needles, instrument pivots, electrical contacts, and compass needles;
  • Osmium tetroxide is often used as a fingerprint detection chemical, as well as in microscopy as a stain for fatty tissue;
  • Synovectomy is a technique in medicine used for treatment of arthritis in which osmium is applied;
  • In transmission electron microscopy (TEM), osmium is used as an intensifier of the image contrast;
  • Together with iridium and rhenium, the 188Os isotope is applied in the dating of both terrestrial and meteoric rocks. 

How Dangerous Is Osmium?

Osmium tetroxide (OsO4) is considered as a highly toxic compound that may cause severe health problems, such as lung decongestion, eye irritation and inflammation, as well as skin damage. 

Upon inhalation of osmium fumes or metal dust particles, the following symptoms of osmium toxicity are typically  experienced:

  • Severe headaches;
  • Loss of breath and difficulty breathing;
  • Wheezing;
  • Abdominal cramps;
  • Lung oedema;
  • Kidney problems;
  • Gastrointestinal problems;
  • Eye damage;
  • Blurred vision;
  • Loss of vision;
  • Skin burn and discoloration;
  • Shock or collapse;
  • Death.

Environmental Effects of Osmium

Osmium tetroxide is the main source of element 76 in the environment. It forms due to the fact that some compounds of this toxic chemical element are converted to tetroxide upon their exposure to air. For this reason, this osmium compound is referred to as an airborne toxin. 

Isotopes of Osmium

Naturally occurring osmium (76Os) is made up of seven isotopes:

  • 184Os (0.02% of abundance);
  • 186Os (1.6% of abundance); 
  • 187Os (2.0% of abundance);
  • 188Os (13.2% of abundance); 
  • 189Os (16.1% of abundance);
  • 190Os (26.3% of abundance);
  • 192Os (40.8% of abundance).

There are 37 observed isotopes of osmium, with atomic masses ranging from osmium-161 to osmium-197. Among them, 30 are synthetically produced radioactive isotopes. Osmium-186 is the longest living form of element 76, with a half-life of 2.0(11)×1015 years. 


[n 1]

ZNIsotopic mass (Da)

[n 2][n 3]


[n 4]



[n 5]



[n 6]

Spin and


[n 7][n 8]

Natural abundance (mole fraction)
Excitation energyNormal proportionRange of variation
161Os7685 0.64(6) msα157W   
162Os7686161.98443(54)#1.87(18) msα158W0+  
163Os7687162.98269(43)#5.5(6) msα159W7/2−#  
β+, p (rare)162W
β+ (rare)163Re
164Os7688163.97804(22)21(1) msα (98%)160W0+  
β+ (2%)164Re
165Os7689164.97676(22)#71(3) msα (60%)161W(7/2−)  
β+ (40%)165Re
166Os7690165.972691(20)216(9) msα (72%)162W0+  
β+ (28%)166Re
167Os7691166.97155(8)810(60) msα (67%)163W3/2−#  
β+ (33%)167Re
168Os7692167.967804(13)2.06(6) sβ+ (51%)168Re0+  
α (49%)164W
169Os7693168.967019(27)3.40(9) sβ+ (89%)169Re3/2−#  
α (11%)165W
170Os7694169.963577(12)7.46(23) sβ+ (91.4%)170Re0+  
α (8.6%)166W
171Os7695170.963185(20)8.3(2) sβ+ (98.3%)171Re(5/2−)  
α (1.7%)167W
172Os7696171.960023(16)19.2(5) sβ+ (98.9%)172Re0+  
α (1.1%)168W
173Os7697172.959808(16)22.4(9) sβ+ (99.6%)173Re(5/2−)  
α (.4%)169W
174Os7698173.957062(12)44(4) sβ+ (99.97%)174Re0+  
α (.024%)170W
175Os7699174.956946(15)1.4(1) minβ+175Re(5/2−)  
176Os76100175.95481(3)3.6(5) minβ+176Re0+  
177Os76101176.954965(17)3.0(2) minβ+177Re1/2−  
178Os76102177.953251(18)5.0(4) minβ+178Re0+  
179Os76103178.953816(19)6.5(3) minβ+179Re(1/2−)  
180Os76104179.952379(22)21.5(4) minβ+180Re0+  
181Os76105180.95324(3)105(3) minβ+181Re1/2−  
182Os76106181.952110(23)22.10(25) hEC182Re0+  
183Os76107182.95313(5)13.0(5) hβ+183Re9/2+  
184Os76108183.9524891(14)Observationally Stable[n 9]0+2(1)×10−4 
185Os76109184.9540423(14)93.6(5) dEC185Re1/2−  
186Os[n 10]76110185.9538382(15)2.0(11)×1015 yα182W0+0.0159(3) 
187Os[n 11]76111186.9557505(15)Observationally Stable[n 12]1/2−0.0196(2) 
188Os[n 11]76112187.9558382(15)Observationally Stable[n 13]0+0.1324(8) 
189Os76113188.9581475(16)Observationally Stable[n 14]3/2−0.1615(5) 
190Os76114189.9584470(16)Observationally Stable[n 15]0+0.2626(2) 
191Os76115190.9609297(16)15.4(1) dβ191Ir9/2−  
192Os76116191.9614807(27)Observationally Stable[n 16]0+0.4078(19) 
193Os76117192.9641516(27)30.11(1) hβ193Ir3/2−  
194Os76118193.9651821(28)6.0(2) yβ194Ir0+  
195Os76119194.96813(54)6.5 minβ195Ir3/2−#  
196Os76120195.96964(4)34.9(2) minβ196Ir0+  
197Os76121 2.8(6) min     

Source: Wikipedia

List of Osmium Compounds 

When osmium participates in a compound, it can occur in the oxidation states of +2 to +8. However, this transition metal most commonly adopts the +2, +3, +4, and +8 oxidation states. 

Osmium forms unstable compounds with alkali elements, while it produces the highly toxic chlorine gas in reaction with hydrochloric acid. Osmium also reacts with base compounds, fluorine, and halogens. 

When osmium powder is exposed to air, osmium tetra-oxide is formed. Osmium(VIII) oxide (OsO4) is a powerful oxidizing agent and highly toxic osmium compound with a pale-yellow color. It carries the characteristic pungent smell associated with this chemical element. 

The list of most commonly prepared osmium compounds contains the following items:

  • Osmocene
  • Osmiridium
  • Osmium borides
  • Osmium dioxide
  • Osmium hexafluoride
  • Osmium pentacarbonyl
  • Osmium pentafluoride
  • Osmium tetroxide
  • Osmium(IV) chloride
  • Potassium osmate
  • Triosmium dodecacarbonyl

5 Interesting Facts and Explanations

  1. 1803 was a fruitful year for English scientists. While Smithson Tennant discovered the chemical elements osmium and iridium the same year, Wollaston also succeeded to discover two other chemicals – palladium and rhodium. 
  2. While performing his chemical experiments and studies, Tennant also became the first scientist to determine that diamonds are pure carbon
  3. Osmium is the densest naturally occurring stable chemical element on Earth. 
  4. Out of all elements that comprise the PMG – platinum metals group (ruthenium, rhodium, palladium, iridium, and platinum), osmium has the highest melting point and the lowest vapor pressure. 
  5. Element 76 has no known biological role.