Rhodium

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45 rutheniumrhodiumpalladium
Co

Rh

Ir
Rh-TableImage.png
General
Name, Symbol, Number rhodium, Rh, 45
Chemical series transition metals
Group, Period, Block 9, 5, d
Appearance silvery white metallic
Rh,45.jpg
Atomic mass 102.90550(2) g/mol
Electron configuration [Kr] 4d8 5s1
Electrons per shell 2, 8, 18, 16, 1
Physical properties
Phase solid
Density (near r.t.) 12.41 g/cm³
Liquid density at m.p. 10.7 g/cm³
Melting point 2237 K
(1964 °C, 3567 °F)
Boiling point 3968 K
(3695 °C, 6683 °F)
Heat of fusion 26.59 kJ/mol
Heat of vaporization 494 kJ/mol
Heat capacity (25 °C) 24.98 J/(mol·K)
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 2288 2496 2749 3063 3405 3997
Atomic properties
Crystal structure cubic face centered
Oxidation states 2, 3, 4
(amphoteric oxide)
Electronegativity 2.28 (Pauling scale)
Ionization energies 1st: 719.7 kJ/mol
2nd: 1740 kJ/mol
3rd: 2997 kJ/mol
Atomic radius 135 pm
Atomic radius (calc.) 173 pm
Covalent radius 135 pm
Miscellaneous
Magnetic ordering no data
Electrical resistivity (0 °C) 43.3 nΩ·m
Thermal conductivity (300 K) 150 W/(m·K)
Thermal expansion (25 °C) 8.2 µm/(m·K)
Speed of sound (thin rod) (20 °C) 4700 m/s
Speed of sound (thin rod) (r.t.) 275 m/s
Shear modulus 150 GPa
Bulk modulus 380 GPa
Poisson ratio 0.26
Mohs hardness 6.0
Vickers hardness 1246 MPa
Brinell hardness 1100 MPa
CAS registry number 7440-16-6
Notable isotopes
Main article: Isotopes of rhodium
iso NA half-life DM DE (MeV) DP
99Rh syn 16.1 d ε - 99Ru
γ 0.089, 0.353,
0.528
-
101mRh syn 4.34 d ε - 101Ru
IT 0.157 101Rh
γ 0.306, 0.545 -
101Rh syn 3.3 y ε - 101Ru
γ 0.127, 0.198,
0.325
-
102mRh syn 2.9 y ε - 102Ru
γ 0.475, 0.631,
0.697, 1.046
-
102Rh syn 207 d ε - 102Ru
β+ 0.826, 1.301 102Ru
β- 1.151 102Pd
γ 0.475, 0.628 -
103Rh 100% Rh is stable with 58 neutrons
105Rh syn 35.36 h β- 0.247, 0.260,
0.566
105Pd
γ 0.306, 0.318 -

Rhodium (chemical symbol Rh, atomic number 45) is a rare, silvery-white, inert metal. It is a member of the platinum group of elements and is found in platinum ores. As the most expensive precious metal,[1] it is a symbol of wealth and is occasionally used in the presentation of high honors.

This metal is a hardening agent for the production of durable, heat-resistant alloys with platinum and palladium. These alloys can be found in such items as furnace windings, thermocouple elements, aircraft spark plugs, and laboratory crucibles. Rhodium is also good for making electrical contacts, and it is a valuable catalyst for automobile catalytic converters and various industrial processes. As a highly reflective metal, it provides a good finish for jewelry, mirrors, and searchlights.

Contents

Occurrence

Rhodium occurs in small amounts in ores of other metals such as platinum, palladium, nickel, silver, and gold. Its industrial extraction and purification processes are therefore complex. Principal sources of this element are located in South Africa, Russia (river sands of the Ural Mountains), and some parts of North and South America. In Canada, it is extracted from the copper-nickel sulfide mining area of the Sudbury, Ontario region. Although the quantity at Sudbury is very small, the large amount of nickel ore processed makes rhodium recovery cost-effective. The main exporter of rhodium is South Africa, followed by Russia.

Rhodium can also be extracted from spent nuclear fuel, which contains an average of 400 grams of rhodium per metric ton. Rhodium from this source contains radioactive isotopes with half-lives of up to 2.9 years. In other words, after every 2.9 years, the radioactivity drops by 50 percent. It is therefore stored for at least 20 years in a secure area, to allow it to become stable.

Discovery

Rhodium (Greek rhodon, meaning "rose") was discovered in 1803 by William Hyde Wollaston, soon after he discovered palladium. Wollaston made this discovery in England using crude platinum ore that presumably came from South America. After performing a series of chemical reactions to extract platinum and palladium from the ore, he obtained a red powder of sodium rhodium chloride (Na3RhCl6.12H2O). He then isolated rhodium metal from the powder by reacting it with hydrogen gas.

Notable characteristics

Rhodium foil and wire

Rhodium is classified as a transition metal. In the periodic table, it lies in period five between ruthenium and palladium and is closely related to the latter two elements. In addition, it is situated in group nine (former group 8B), between cobalt and iridium.

This hard, silvery metal is highly reflective and extremely resistant to corrosion. It has a higher melting point and lower density than platinum. It is not attacked by most acids and dissolves only in aqua regia (a mixture of concentrated hydrochloric acid and nitric acid in the ratio 3:1 by volume). If slowly cooled from a red-hot state in the presence of air, rhodium changes to the sesquioxide, and the latter converts back to the metal at higher temperatures.

Isotopes

Naturally occurring rhodium is composed of only one isotope, 103Rh, which is stable. In addition, many radioactive isotopes, ranging in mass numbers from 89 to 122, have been produced artificially. Of these, the radioisotopes with the longest half-lives are: 101Rh, with a half-life of 3.3 years; 102Rh, with a half-life of 2.9 years; 102mRh, with a half-life of 207 days; and 99Rh, with a half-life of 16.1 days.

Applications

  • The primary use of rhodium is as an alloying agent for hardening platinum and palladium. These alloys are used in furnace windings, bushings for glass fiber production, thermocouple elements, aircraft turbine engines and spark plugs, and laboratory crucibles.
  • Rhodium is also used as an electrical contact material due to its low electrical resistance, low and stable contact resistance, and its high corrosion resistance.
  • Plated rhodium, made by electroplating or evaporation, is extremely hard and is used for optical instruments.
  • Given its highly reflective nature, this metal is used as a finish for jewelry, decorative items, mirrors, and search lights.
  • Rhodium is an important catalyst in automobile catalytic converters and a number of industrial processes (such as the manufacture of acetic acid from methanol). It also catalyzes a process (addition of hydrosilanes to a double bond) used for the manufacture of certain silicone rubbers.
  • The complex of a rhodium ion with the organic compound called “BINAP” gives a widely used catalyst for certain organic chemical syntheses.

Chlorides of rhodium

The name rhodium(III) chloride usually refers to hydrated rhodium trichloride, a molecular compound with the formula RhCl3(H2O)3. Another prominent rhodium chloride is RhCl3, a polymeric solid that behaves quite differently. Most chemistry ascribed to "rhodium trichloride" refers to the use of the hydrated form. Some procedures calling for a rhodium chloride imply the use of Na3RhCl6. These chlorides are the products of the separation of rhodium from the other platinum group metals.

RhCl3(H2O)3 exists as dark red crystals. It is soluble in water to give reddish solutions. It is used to prepare a variety of complexes, such as with carbon monoxide, alkenes, pyridine, and phosphines. RhCl3(H2O)3 and some of its complexes are useful catalysts for various reactions. For example, the complex with phosphine (RhCl(PPh3)3), well-known as Wilkinson’s catalyst, is used for the hydrogenation and isomerization of alkenes.

Precautions

As a noble metal, rhodium is chemically inert. Its compounds, however, can be reactive and should be considered highly toxic and carcinogenic. For rats, the lethal intake (LD50) of rhodium chloride (RhCl3) was found to be 12.6 milligrams per kilograms (mg/kg). Rhodium compounds can leave strong stains on human skin. The element is not known to play any biological role in humans.

Symbolism

Rhodium symbolizes wealth and has been used for giving high honors when more commonly used metals such as silver, gold, or platinum are deemed insufficient. In 1979, the Guinness Book of World Records gave Paul McCartney a rhodium-plated disc for being history's all-time best-selling songwriter and recording artist. Guinness has also noted items such as the world's "Most Expensive Pen" or "Most Expensive Board Game" as containing rhodium.

See also

Notes

  1. Tax Free Gold – Precious Metal Prices Index Retrieved December 4, 2007.

References

  • Canterford, J.H. and R. Colton. 1968. Halides of the Second and Third Row Transition Metals. London: Wiley-Interscience.
  • Cotton, S.A. 1997. Chemistry of the Precious Metals. Chapman and Hall. ISBN 0-7514-0413-6
  • Greenwood, N.N. and Earnshaw, A. 1998. Chemistry of the Elements, 2nd Edition. Oxford, U.K.; Burlington, Massachusetts: Butterworth-Heinemann, Elsevier Science. ISBN 0750633654
  • Rhodium Los Alamos National Laboratory. Retrieved December 4, 2007.
  • Rhodium WebElements.com. Retrieved December 4, 2007.

External links

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