Osmium is the densest of all natural elements: 250 ml of this metal (in its liquid form) weighs 5.5 kg. This rare element is found in the mineral osmiridium. Pure osmium reacts with oxygen in air to form a poisonous oxide (osmium tetroxide, OsOsub>4), so the metal is used safely in combination with other elements or alloys. OsOsub>4 is used to stain cells red so that they can be seen clearly under a microscope. Also, black osmium oxide powder is used as a fingerprint developer. In addition, it is used in fountain pen nibs because of its hardness in alloys.

Summary of properties (Os)

Atomic weight190.23(3)
Discoverer (year)Tennant, Smithson (1803)
Natural formmetallic solid (hexagonal)
Electron configuration[Xe] 414 5d6 6s2
Melting point (ºC)3045
Boiling point (ºC)5027
Abundance in earth's crust (ppm)0.002
Isotope (abundance)184Os (0.02), 186Os (1.59), 187Os (1.96), 188Os (13.24), 189Os (16.15), 190Os (26.26), 192Os (40.78)
Density g/cm322.6
Van der Waals radius (pm)216
Covalent radius (pm)136
Electronegativity (Pauling)2.2
Vaporization enthalpy (kJ/mol)627.60
Enthalpy of fusion (kJ/mol)57.85
Specific heat capacity (J/g·K) at 25ºC and 1 atm0.13
Thermal conductivity (W/cm·K) at 25ºC and 1 atm0.880
Oxidation state+4, +3
Electron affinity (eV)1.10
1st Ionization potential (eV)8.4382

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