Rhodium Element Facts


Rhodium

Rhodium pin and foil. Photo by Dschwen.

45
Rh
102.9

Data Zone

Classification: Rhodium is a transition metal
Color: silvery-white
Atomic weight: 102.9055
State: solid
Melting point: 1963 oC, 2236 K
Boiling point: 3695 oC, 3968 K
Electrons: 45
Protons: 45
Neutrons in most abundant isotope: 58
Electron shells: 2,8,18,16,1
Electron configuration: [Kr] 4d8 5s1
Density @ 20oC: 12.4 g/cm3
Show more, including: Heats, Energies, Oxidation, Reactions, Compounds, Radii, Conductivities

Atomic volume: 8.3 cm3/mol
Structure: fcc: face-centered cubic
Hardness: 6.0 mohs
Specific heat capacity 0.242 J g-1 K-1
Heat of fusion 21.50 kJ mol-1
Heat of atomization 556 kJ mol-1
Heat of vaporization 493.0 kJ mol-1
1st ionization energy 719.8 kJ mol-1
2nd ionization energy 1744.4 kJ mol-1
3rd ionization energy 2996.8 kJ mol-1
Electron affinity 109.7 kJ mol-1
Minimum oxidation number -1
Min. common oxidation no. 0
Maximum oxidation number 6
Max. common oxidation no. 3
Electronegativity (Pauling Scale) 2.28
Polarizability volume 8.6 Å3
Reaction with air mild, w/ht, ⇒ Rh2O3
Reaction with 15 M HNO3 none
Reaction with 6 M HCl none
Reaction with 6 M NaOH
Oxide(s) RhO2, Rh2O3
Hydride(s)
Chloride(s) RhCl3
Atomic radius 134 pm
Ionic radius (1+ ion)
Ionic radius (2+ ion)
Ionic radius (3+ ion) 80.5 pm
Ionic radius (1- ion)
Ionic radius (2- ion)
Ionic radius (3- ion)
Thermal conductivity 150 W m-1 K-1
Electrical conductivity 23 x 106 S m-1
Freezing/Melting point: 1963 oC, 2236 K


The Platinum Group Metals
These metals have similar properties and are often present in the same mineral ores.

Discovery of Rhodium

Rhodium was discovered in 1803, in London, by English chemist William H. Wollaston, when examining a platinum ore from Peru. (1a)

Wollaston was first alerted to the possibility of a new element by Hippolyte-Victor Collet-Descotils, who believed that the red color of some platinum salts was actually caused by the presence of an unidentified metal. (2)

To investigate this possibility, Wollaston first dissolved crude platinum in aqua regia, a concentrated solution of hydrochloric and nitric acids. He then precipitated platinum metal by dissolving the solution in ammonium chloride.

The liquid which remained had properties which matched no known substance. (2)

Wollaston conducted a series of chemical reactions on this liquid before producing a deep red powder, sodium rhodium chloride, RhCl6Na3.12H2O, which gave a black, flaky precipitate of rhodium when treated with zinc. (1b)

The element name comes from the Greek word ‘rhodon’ meaning rose. Wollaston chose this name because of the rose color of a dilute solution of rhodium’s salts.

1803 was a good year for William Wollaston. Not only did he discover rhodium, he also discovered another platinum group metal – palladium.

Rhodium catalyst for ethanol oxidation.

Catalyst for ethanol oxidation: platinum-rhodium clusters on a surface of tin dioxide. This catalyst can split the carbon-carbon bond and obtain energy from ethanol in fuel cells. Image by BNL. (3)

Appearance and Characteristics

Harmful effects:

Rhodium is considered to be non-toxic. Some of its compounds are highly toxic and carcinogenic.

Characteristics:

Rhodium is a rare, hard, silvery-white, lustrous metal.

It is one of the of the six platinum group metals consisting of platinum, palladium, rhodium, osmium, iridium and ruthenium.

It is highly reflective and extremely resistant to corrosion.

It is not attacked by most acids.

When present in compounds, rhodium exists mostly in the trivalent state, Rh3+.

Rhodium’s salts form rose-colored aqueous solutions.

Uses of Rhodium

The majority of rhodium is used as a catalyst in catalytic converters. It is also used to catalyze industrial processes.

Rhodium is used as an alloying agent for hardening and improving the corrosion resistance of platinum and palladium.

As a result of its low electrical resistance, low and stable contact resistance, and its stability against corrosion rhodium is used as an electrical contact material.

The metal is used in jewelry and for decorations.

Abundance and Isotopes

Abundance earth’s crust: 1 part per billion by weight, 0.1 parts per billion by moles

Abundance solar system: 2 parts per billion by weight, 0.02 parts per billion by moles

Cost, pure: $13,000 per 100g

Cost, bulk: $7,000 per 100g

Source: Rhodium occurs in small quantities in ores metals such as platinum, palladium, nickel, silver, and gold. Commercially, it is obtained as a byproduct of refining nickel sulfide ores from Canada.

Isotopes: Rhodium has 24 isotopes whose half-lives are known, with mass numbers from 94 to 117. Naturally occurring rhodium consists of its one stable isotope, 103Rh.

References

1a. J.W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry Volume XV, 1936, Longmans, Green and Co., p545
1b. J.W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry Volume XV, 1936, Longmans, Green and Co., p546
2. William Nicolson, A Journal of Natural Philosophy, Chemistry and the Arts, Volumes 9-10, 1804, W. Stratford, p34-35
3. Photo: BNL

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