Periodic Table


Atomic number: 76
Atomic weight: 190.23
Symbol: Os
Group number: 8
Electronic configuration: [Xe].4f14.5d6.6s2


Osmium metal is lustrous, bluish white, extremely hard, and brittle even at high temperatures. It has the highest melting point and lowest vapour pressure of the platinum group. The metal is very difficult to fabricate, but the powder can be sintered in a hydrogen atmosphere at a temperature of 2000°C. The solid metal is not affected by air at room temperature, but the powdered or spongy metal slowly gives off osmium tetroxide, which is a powerful oxidising agent and has a strong smell. The tetroxide is highly toxic, and boils at 130°C (760 mm). Concentrations in air as low as 10-7 g m-3 can cause lung congestion, skin damage, or eye damage.

General information

Discoveror: Smithson Tennant
Date discovered: 1803
Discovered at: England
Meaning of name: From the Greek word "osme" meaning "smell"

Physical data

Standard state: solid at 298 K
Colour: bluish grey
Density of solid at ambient temperature/kg m-3: 22610
Molar volume/cm3: 8.42

Radii /pm

Atomic: 185
Covalent (single bond):
Pauling radius for the ion [Os]-: no data

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius155.5no data70.621.3


Both values are quoted on the Pauling scale.

Pauling: 2.2
Allred Rochow: 1.52

Crystal Structure

structure: hcp (hexagonal close-packed)

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 3306
boiling point: 5285

Enthalpies /kJ mol-1

fusion: 31.0
vaporization: 628

single bond enthalpies:
Os-F Os-Cl Os-Br Os-I Os-Os
no data no data no data no data no data

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance
184Os183.952488 (4)0.02 (1)
186Os185.953830 (4)1.58 (30)
187Os186.955741 (3)1.6 (1)
188Os187.955830 (3)13.3 (2)
189Os188.958137 (4)16.1 (3)
190Os189.958436 (4)26.4 (4)
192Os191.961467 (4)41.0 (3)

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/Os.html