Osmium (Os)

Stable isotopes of osmium available from ISOFLEX

Isotope Z(p) N(n) Atomic Mass Natural Abundance Enrichment Level Chemical Form
Os-184 76  108  183.952491 0.02%  ≥96.90% Metal
Os-186  76  110  185.953838 1.59%  >99.00% Metal
Os-187 76 111 186.955748 1.96% >99.00% Metal
Os-188 76 112 187.955836 13.24% ≥94.00% Metal
Os-189 76 113 188.958145 16.15% >99.00% Metal
Os-190 76 114 189.958445 26.26% >99.00% Metal
Os-192 76 116 191.961479 40.78% >99.00% Metal

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Os

Osmium was discovered in 1803 by Smithson Tennant. Its name derives from the Greek word osme, meaning “smell.”

A bluish-white, lustrous, brittle and fairly hard metal of the platinum group, osmium has a close-packed hexagonal system. On heating in air, it gives off the poisonous fume of osmium tetroxide. It has the highest specific gravity and melting point of the platinum metals. It is metallurgically unworkable and has a magnetic susceptibility of 0.052 x 10-6 cm3/g. It is insoluble in water, HCl, H2SO4 and ammonia; slightly soluble in nitric acid and aqua regia; and solubilized by fusion with caustic soda, sodium peroxide, potassium chlorate and the mass dissolved in water. In its finely divided form, it reacts slowly with oxygen or air at ambient temperatures to form osmium tetroxide. The bulk metal is stable in oxygen at ordinary temperatures but reacts at 200 ºC, forming osmium tetroxide. Osmium is stable in mineral acids, even under boiling conditions. The metal, however, is attacked by fused alkalis such as caustic soda and caustic potash, particularly in the presence of an oxidizing agent such as sodium peroxide, sodium hypochlorite or sodium nitrite, forming osmates.

The commercial applications of osmium are limited and considerably fewer than those of other platinum group metals. Its alloys are very hard and are used to make tips of fountain-pen nibs, phonograph needles and pivots. The metal also exhibits effective catalytic properties in hydrogenation and other organic reactions. Such catalytic applications, however, are limited; osmium fails to replace other noble metals, particularly palladium and platinum, which cost less and are more effective as catalysts.

Properties of Osmium

Name Osmium
Symbol Os 
Atomic number 76 
Atomic weight 190.2 
Standard state Solid at 298 ºK 
CAS Registry ID 7440-04-2 
Group in periodic table 8 
Group name Precious metal or platinum group metal 
Period in periodic table 6 
Block in periodic table d-block 
Color Bluish-gray 
Classification Metallic
Melting point 3050 °C
Boiling point 5027 °C
Vaporization point 5300 ºC
Electrical resistivity 9.5 µΩ·cm at 20 ºC 
Electronegativity 2.2 
Specific heat 0.131 kJ/kg K at 20 ºC
Heat of vaporization 630 kJ·mol-1 at 5027 ºC
Heat of fusion 31 kJ·mol-1
Density of liquid 20 g/cm3 at 3050 °C 
Density of solid 22.48 g/cm3
Electron configuration [Xe]4f145d66s2 
Oxidation states  +1, +2, +3, +4, +5, +6, +7, +8 
Most common oxidation states +3, +4, +6
Moh’s hardness scale 7.0

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