OSMIUM

TRANSITION ELEMENT: IRON GROUP

Atomic number:	76
Group numbers:	8
Period:	6
Electronic configuration:	[Xe] 4f¹⁴ 5d⁶ 6s²
Formal oxidation number:	+3 +4
Electronegativities:	2.2
Atomic radius / pm:	133.8
Relative atomic mass:	190.23(3)

Osmium was discovered by Smithson Tennant (GB) in 1803. The origin of the name comes from the Greek word osme meaning smell. It is a hard fine black powder or hard, lustrous, blue-white metal. Osmium is unaffected by air, water and acids. It has a characteristic acrid, chlorine like odour due to tetroxide compound. Osmium tetroxide is highly toxic. Osmium is obtained from the same ores as platinum. It is used to tip gold pen points, instrument pivots, to make electric light filaments, and also used for high temperature alloys and pressure bearings. Osmium is very hard and resists corrosion better than any other element. The price of 99.95 % pure osmium powder is 2159.60 € for 25 g.

PHYSICAL PROPERTIES
Density / g dm^-3:	22590	(293 K)
	20100	(m.p.)
Molar volume / cm³mol^-1:	8.42	(293 K)
	9.46	(m.p.)
Electrical resistivity / µΩcm:	9.5	(20 °C)

THERMAL PROPERTIES
Thermal conductivity / W m^-1K^-1:	87.6
Melting point / °C:	3033
Boiling point / °C:	5012
Heat of fusion / kJ mol^-1:	29.3
Heat of vaporization / kJ mol^-1:	738.06
Heat of atomization / kJ mol^-1:	791

IONIZATION ENERGIES
First ionization energy / kJ mol^-1:	814.17
Second ionization energy / kJ mol^-1:	-
Third ionization energy / kJ mol^-1:	-

ABUNDANCE OF ELEMENT
in the atmosphere / ppm:	-
in the Earth's crust / ppm:	0.005
in the oceans / ppm:	-

CRYSTALLOGRAPHIC DATA
Crystal structure:	hexagonal
Unit-cell dimensions / pm:	a=273.43, c=432.00
Space group:	P6₃/mmc

ISOTOPES
Isotope	Relative atomic mass	Mass percent (%)
¹⁸⁴Os	183.952491(3)	0.02(1)
¹⁸⁶Os	185.953838(3)	1.58(30)
¹⁸⁷Os	186.955748(3)	1.6(3)
¹⁸⁸Os	187.955836(3)	13.3(7)
¹⁸⁹Os	188.958145(3)	16.1(8)
¹⁹⁰Os	189.958445(3)	26.4(12)
¹⁹²Os	191.961479(4)	41.0(8)

REDUCTION POTENTIALS
Balanced half-reaction	E^o / V
Os²⁺ + 2e^- → Os(s)	+0.85	(1 mol dm^-3 HCl)
OsO₂(s) + 2H₂O + 4e^- → Os(s) + 4OH^-	- 0.15
OsO₄(s) + 8H⁺ + 8e^- → Os(s) + 4H₂O	+0.85
OsO₄(s) + 8H⁺ + 6Cl^- + 4e^- → OsCl₆^2- + 4H₂O	+1.0
HOsO₅^- + 2e^- → OsO₄^2- + OH^-	+0.3
OsO₄^2- + 2H₂O + 2e^- → OsO₂(s) + 4OH^-	+0.1
OsCl₆^2- + e^- → OsCl₆^3-	+0.85
OsCl₆^3- + e^- → Os²⁺ + 6Cl^-	+0.4
OsCl₆^3- + 3e^- → Os(s) + 6Cl^-	+0.71