Rhodium

45

ruthenium ← rhodium → palladium


Co
↑
Rh
↓
Ir




periodic table




General


Name, Symbol, Number

rhodium, Rh, 45


Chemical series
transition metals


Group, Period, Block

9, 5, d


Appearance
silvery white metallic 



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.

Contents

1 Occurrence
2 Discovery
3 Notable characteristics

3.1 Isotopes


4 Applications
5 Chlorides of rhodium
6 Precautions
7 Symbolism
8 See also
9 Notes
10 References
11 External links
12 Credits



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.

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
Chemical element
Metal
Periodic table
Transition metal
Notes

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


ReferencesISBN links support NWE through referral fees
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
All links retrieved December 8, 2022.

Kitco Rhodium prices
EnvironmentalChemistry.com – Rhodium
Lenntech – Rhodium

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