Atomic Number: 76 Atomic Radius: 216 pm (Van der Waals)
Atomic Symbol: Os Melting Point: 3033 °C
Atomic Mass: 190.2 Boiling Point: 5012 °C
Electron Configuration:
[Xe] 6s2 4f14 5d6
Oxidation States: 3, 4
Smithson Tennant
Osmium was discovered by Smithson Tennant in 1803.


(Gr. osme, a smell) Discovered in 1803 by Tennant in the residue left when crude platinum is dissolved by aqua regia.


Osmium occurs in iridosule and in platinum-bearing river sands in the Urals, North America, and South America. It is also found in the nickel-bearing ores of Sudbury, Ontario region along with other platinum metals. While the quantity of platinum metals in these ores is very small, the large tonnages of processed nickel ores make commercial recovery possible.



The metal is lustrous, bluish white, extremely hard, and brittle even at high temperatures. It has the highest melting point and the lowest vapor pressure of the platinum group. The metal is very difficult to fabricate, but the powdered or spongy metal slowly gives off osmium tetroxide, which as a powerful oxidizing agent and has a strong smell. The tetroxide is highly toxic, and boils at 130oC.

The measured densities of iridium and osmium seem to indicate that osmium is slightly more dense than iridium, so osmium has generally been credited with being the heavier element. Calculations of the density from the space lattice which may be more reliable for these elements than actual measurements, however, give a density of 22.65 for iridium compared to 22.661 for osmium. Despite this information, no decision has been made as to which is heavier.

Osmium - fountain pen
Osmium is used to produce fountain pen tips.


Concentrations in air as low as 107 g/m3 can cause lung congestion, skin damage, or eye damage. Exposure to osmium tetroxide should not exceed 0.0016 mg/m3 (8-hour time weighted average – 40-hour work week).


The tetroxide has been used to detect fingerprints and to stain fatty tissue for microscope slides. The metal is almost entirely used to produce very hard alloys with other metals of the platinum group for fountain pen tips, instrument pivots, phonograph needles, and electrical contacts.


Los Alamos National Laboratory, Periodic Table of Elements, http://periodic.lanl.gov/list.shtml.