76 Os 190.23

Osmium

Transition metal Osmium solid

[Xe] 4f¹⁴ 5d⁶ 6s² · 2 · 8 · 18 · 32 · 14 · 2

← Re Rhenium Ir Iridium →

The densest of all the elements: a cubic centimetre of osmium weighs 22.59 grams. It is also extremely brittle, and its tetroxide is volatile, poisonous and sharply pungent — hence the name, from the Greek for smell.

3D model

Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.

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Atomic properties

Atomic number: 76
Atomic mass: 190.23 а.е.м.
Electron configuration: [Xe] 4f¹⁴ 5d⁶ 6s²
Electrons per shell: 2 · 8 · 18 · 32 · 14 · 2
Block: d-block
Group: Iron group
Period: 6
Electronegativity (Pauling): 2.2
Electronegativity (Allen): 1.65
Atomic radius: 185 pm
Covalent radius: 144 pm
Van der Waals radius: 216 pm
Ionisation energy 1: 840 kJ/mol
Ionisation energy 2: 1 600 kJ/mol
Electron affinity: 106.1 kJ/mol
Common oxidation states: +4
Oxidation states: -4, -2, -1, +1, +2, +3, +4, +5, +6, +7, +8

Physical properties

State at 25 °C: solid
Density: 22.59 g/cm³
Melting point: 3 306 K · 3 032.9 °C
Boiling point: 5 285 K · 5 011.9 °C
Speed of sound: 4 940 m/s

Thermal properties

Heat of fusion: 57.85 kJ/mol
Heat of vaporisation: 738 kJ/mol
Specific heat capacity: 0.13 J/(g·K)
Thermal conductivity: 87.6 W/(m·K)

Mechanical properties

Young's modulus: 550 GPa
Shear modulus: 222 GPa
Bulk modulus: 462 GPa
Poisson ratio: 0.25
Mohs hardness: 7
Brinell hardness: 3 920 MPa

Electrical and magnetic properties

Electrical resistivity: 81.2 nΩ·m
Magnetic ordering: paramagnetic
Curie point: no data
Néel point: no data
Superconducting point: 0.66 K

Crystal structure

Crystal structure: hexagonal close-packed (hcp)
Lattice constants: a = 273.44 pm · c = 431.73 pm

Abundance

In the crust: 0.0001 mg/kg
In the ocean: no data
In the universe: 0.003 mg/kg
In the human body: no data

Isotopes

Isotope	Abundance	Half-life	Decay mode
¹⁸⁴Os	0.02 %	stable	—
¹⁸⁶Os	1.59 %	63.38 Myr	α
¹⁸⁷Os	1.96 %	stable	—
¹⁸⁸Os	13.24 %	stable	—
¹⁸⁹Os	16.15 %	stable	—
¹⁹⁰Os	26.26 %	stable	—
¹⁹²Os	40.78 %	stable	—

Osmium-186 alpha-decays with a 2·10¹⁵-year half-life. Osmium-187 accumulates from rhenium-187 decay and underlies rhenium-osmium geochronology.

Discovery

Year of discovery: 1803
Discovered by: Smithson Tennant
Where: England
Origin of the name: from Greek osme (smell): osmium tetroxide has a pungent odour

History

Smithson Tennant examined the black residue left after dissolving platinum in aqua regia in 1803 and found two new elements in it at once: osmium and iridium.

Where it occurs

0.0001 mg/kg of the crust. It occurs in platinum placers and in the natural alloy osmiridium.

How it is produced

From the slimes of platinum and nickel refining. World output is under a tonne a year.

Role in living things

No biological role.

Safety

The metal is inert, but osmium tetroxide is extremely dangerous: its vapour damages the cornea and lungs at concentrations below the threshold of smell.

Uses

Osmium-iridium alloys for the nibs of expensive pens and pivots of precision instruments
Osmium tetroxide as a stain for electron microscopy specimens
Asymmetric dihydroxylation catalysts in fine organic synthesis
Exceptionally wear-resistant contacts

Curiosities

Osmium is the densest known substance under ordinary conditions, just ahead of iridium.
Osmium tetroxide smells sharp enough that the element was named for the odour.
Anomalously high iridium and osmium in the Cretaceous-Palaeogene boundary layer was the key argument for the asteroid theory of the dinosaur extinction.
Osmium is so brittle it cannot be forged: it crumbles even at room temperature.