Osmium

2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements

76 rhenium ← osmium → iridium Ru ↑ Os ↓ Hs Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	osmium, Os, 76
Chemical series	transition metals
Group, Period, Block	8, 6, d
Appearance	silvery, blue cast
Standard atomic weight	190.23 (3)  g·mol−1
Electron configuration	[Xe] 4f14 5d6 6s2
Electrons per shell	2, 8, 18, 32, 14, 2
Physical properties
Phase	solid
Density (near r.t.)	22.61  g·cm−3
Liquid density at m.p.	20  g·cm−3
Melting point	3306  K (3033 ° C, 5491 ° F)
Boiling point	5285  K (5012 ° C, 9054 ° F)
Heat of fusion	57.85   kJ·mol−1
Heat of vaporization	738   kJ·mol−1
Specific heat capacity	(25 °C) 24.7  J·mol−1·K−1
Vapor pressure P(Pa) 1 10 100 1 k 10 k 100 k at T(K) 3160 3423 3751 4148 4638 5256
Atomic properties
Crystal structure	hexagonal
Oxidation states	8, 7, 6, 5, 4, 3, 2, 1, −1, −2 (mildly acidic oxide)
Electronegativity	2.2 (Pauling scale)
Ionization energies	1st: 840 kJ/mol
	2nd: 1600 kJ/mol
Atomic radius	130   pm
Atomic radius (calc.)	185  pm
Covalent radius	128  pm
Miscellaneous
Magnetic ordering	?
Electrical resistivity	(0 °C) 81.2 nΩ·m
Thermal conductivity	(300 K) 87.6  W·m−1·K−1
Thermal expansion	(25 °C) 5.1  µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 4940 m/s
Shear modulus	222  GPa
Poisson ratio	0.25
Bulk modulus	462  GPa
Mohs hardness	7.0
Brinell hardness	3920  MPa
CAS registry number	7440-04-2
Selected isotopes
Main article: Isotopes of osmium iso NA half-life DM DE ( MeV) DP 184Os 0.02% >5.6×1013 y (not observed) εε 1.452 184W 185Os syn 93.6 d ε 1.013 185Re 186Os 1.59% 2.0×1015 y α 2.822 182W 187Os 1.96% 187Os is stable with 111 neutrons 188Os 13.24% 188Os is stable with 112 neutrons 189Os 16.15% 189Os is stable with 113 neutrons 190Os 26.26% 190Os is stable with 114 neutrons 191Os syn 15.4 d β- 0.314 191Ir 192Os 40.78% >9.8×1012 y (not observed) ββ 0.414 192Pt 193Os syn 30.11 d β- 1.141 193Ir 194Os syn 6 y β- 0.097 194Ir
References

Osmium (pronounced /ˈɒzmiəm/) is a chemical element that has the symbol Os and atomic number 76. Osmium is a hard, brittle, blue-gray or blue-black transition metal in the platinum family, and is one of the densest natural elements, competing for this status with iridium. Osmium is used in alloys with platinum, iridium and other platinum group metals. Osmium is found in nature as an alloy in platinum ore. Alloys of osmium are employed in fountain pen tips, electrical contacts and in other applications where extreme durability and hardness are needed.

Notable characteristics

Osmium in a metallic form is extremely dense, blue-white, brittle, and lustrous even at high temperatures, but proves to be extremely difficult to make. Powdered osmium is easier to make, but powdered osmium exposed to air leads to the formation of osmium tetroxide (OsO₄), which is very toxic. The tetroxide is a powerful oxidizing agent, very volatile, water-soluble, pale yellow, crystalline solid with a strong smell that boils at 130° C. By contrast osmium dioxide (OsO₂) is black, non-volatile and much less reactive and toxic.

Due to its very high density osmium is generally considered to be the densest known element, narrowly defeating iridium. However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22650 kg/m³ for iridium versus 22610 kg/m³ for osmium. Definitive selection between the two is therefore not possible at this time. If one distinguishes different isotopes, then the highest density ordinary substance would be ¹⁹²Os. The extraordinary density of osmium is a consequence of the lanthanide contraction.

Osmium has a very low compressibility. Correspondingly, its bulk modulus is extremely high—commonly quoted as 462 GPa, which is higher than that of diamond but lower than that of aggregated diamond nanorods—although there is some debate in the academic community about whether it is in fact this high. A paper by Cynn et al reported that osmium had this bulk modulus, based on an experimental result, but other authors have cast doubt upon this ( and references therein).

Osmium metal has the highest melting point and the lowest vapor pressure of the platinum family. Common oxidation states of osmium are +4 and +3, but oxidation states from +1 to +8 are observed.

Applications

Because of the volatility and extreme toxicity of its oxide, osmium is rarely used in its pure state, and is instead often alloyed with other metals that are used in high-wear applications. Osmium alloys such as osmiridium are very hard and, along with other platinum group metals, is almost entirely used in alloys employed in the tips of fountain pens, phonograph needles, instrument pivots, and electrical contacts, as they can resist wear from frequent use.

Osmium tetroxide has been used in fingerprint detection and in staining fatty tissue for microscope slides. As a strong oxidant, it cross-links lipids mainly by reacting with unsaturated carbon-carbon bonds, and thereby both fixes biological membranes in place in tissue samples and simultaneously stains them, since osmium atoms are extremely electron dense, making OsO₄ an important stain for transmission electron microscopy (TEM) studies of many biological materials. An alloy of 90% platinum and 10% osmium (90/10) is used in surgical implants such as pacemakers and replacement pulmonary valves.

The tetroxide (and a related compound, potassium osmate) are important oxidants for chemical synthesis, despite being very poisonous.

In 1898 an Austrian chemist, Auer von Welsbach, developed the Oslamp with a filament made of osmium, which he introduced commercially in 1902. After only a few years, osmium was replaced by the more stable metal tungsten (originally known as wolfram). Tungsten has the highest melting point of any metal, and using it in light bulbs increases the luminous efficacy and life of incandescent lamps.

The light bulb manufacturer OSRAM (founded in 1906 when three German companies; Auer-Gesellschaft, AEG and Siemens & Halske combined their lamp production facilities), derived its name from the elements of OSmium and wolfRAM.

Like palladium, powdered osmium will densely absorb hydrogen atoms, perhaps making it a potential candidate as a metal hydride battery electrode substance, but it will react with potassium hydroxide, the most common battery electrolyte.

History

Osmium ( Greek osme meaning "a smell") was discovered in 1803 by Smithson Tennant and William Hyde Wollaston in London, England.

Wollaston and Tennant were looking for a way to purify platinum by dissolution of native platinum ore in aqua regia. Large amounts of insoluble black powder remained as a byproduct of this operation.

Wollaston concentrated on the soluble portion and discovered palladium (in 1802) and rhodium (in 1804), while Tennant examined the insoluble residue. In the summer of 1803, Tennant identified two new elements, osmium and iridium. Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.

Occurrence

Turkey, with 127,000 tons, has the world's largest known reserve of osmium. Bulgaria also has substantial reserves of about 2500 tons. This transition metal is also found in iridiosmium, a naturally occurring alloy of iridium and osmium, and in platinum-bearing river sands in the Ural Mountains, and North and South America. It also occurs in nickel-bearing ores found in the Sudbury, Ontario region with other platinum group metals. Even though the quantity of platinum metals found in these ores is small, the large volume of nickel ores processed makes commercial recovery possible.

Compounds

Osmium(III) chloride OsCl₃

Osmium(IV) oxide OsO₂

Osmium(VIII) oxide OsO₄

Osmium carbonyl Os₃(CO)₁₂

Isotopes

Osmium has seven naturally occurring isotopes, 6 of which are stable: ¹⁸⁴Os, ¹⁸⁷Os, ¹⁸⁸Os, ¹⁸⁹Os, ¹⁹⁰Os, and (most abundant) ¹⁹²Os. ¹⁸⁶Os undergoes alpha decay with enormously long half-life of (2.0±1.1)×10¹⁵ yr and for many practical purposes can be considered to be stable as well. Alpha decay is predicted for all 7 naturally occurring isotopes, but due to very long half-lives, it was observed only for ¹⁸⁶Os. It is predicted also that ¹⁸⁴Os and ¹⁹²Os can undergo double beta decay but this radioactivity is not still observed.

¹⁸⁷Os is the daughter of ¹⁸⁷Re ( half-life 4.56×10¹⁰ years) and is used extensively in dating terrestrial as well as meteoric rocks (see Rhenium-osmium dating). It has also been used to measure the intensity of continental weathering over geologic time and to fix minimum ages for stabilization of the mantle roots of continental cratons. This decay is a reason that in some minerals rich by rhenium the isotopic abundance of ¹⁸⁷Os is much more than the standard one. However, the most notable application of Os in dating has been in conjunction with iridium, to analyze the layer of shocked quartz along the K-T boundary that marks the extinction of the dinosaurs 65 million years ago.

Precautions

Osmium tetroxide is highly volatile, penetrates skin readily, and is very toxic by inhalation, in contact with skin and if swallowed. Airborne low concentrations of osmium vapour can cause lung congestion, skin or eye damage, and it should therefore be used in a fume hood. Osmium tetroxide is rapidly reduced to relatively inert compounds by poly-unsaturated vegetable oils such as corn oil.