45 ruthenium ← rhodium → palladium Co ↑ Rh ↓ Ir Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	rhodium, Rh, 45
Element category	transition metals
Group, Period, Block	9, 5, d
Appearance	silvery white metallic
Standard atomic weight	102.90550(2)  g·mol−1
Electron configuration	Kr 4d8 5s1
Electrons per shell	2, 8, 18, 16, 1
Physical properties
Phase	solid
Density (near r.t.)	12.41  g·cm−3
Liquid density at m.p.	10.7  g·cm−3
Melting point	2237 K (1964 °C, 3567 °F)
Boiling point	3968 K (3695 °C, 6683 °F)
Heat of fusion	26.59  kJ·mol−1
Heat of vaporization	494  kJ·mol−1
Specific heat capacity	(25 °C) 24.98  J·mol−1·K−1
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	4, 3, 2, 11 (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−1·K−1
Thermal expansion	(25 °C) 8.2  µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 4700 m/s
Young's modulus	380  GPa
Shear modulus	150  GPa
Bulk modulus	275  GPa
Poisson ratio	0.26
Mohs hardness	6.0
Vickers hardness	1246  MPa
Brinell hardness	1100  MPa
CAS registry number	7440-16-6
Most-stable 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% 103Rh is stable with 58 neutrons 105Rh syn 35.36 h β- 0.247, 0.260, 0.566 105Pd γ 0.306, 0.318 -
References

Rhodium (pronounced /ˈroʊdiəm/) is a chemical element with the symbol Rh and atomic number 45. A rare, silvery-white hard transition metal and a member of the platinum group, rhodium is found in platinum ores and is used in alloys with platinum and as a catalyst. It is typically the most expensive precious metal²³

Characteristics

Rhodium is a hard silvery white and durable metal that has a high reflectance. Rhodium metal does not normally form an oxide, even when heated. Oxygen is absorbed from the atmosphere at the melting point of rhodium, but on solidification the oxygen is released.⁴ Rhodium has both a higher melting point and lower density than platinum. It is not attacked by acids: it is completely insoluble in nitric acid and dissolves slightly in aqua regia.

Chemical

See also: Category:Osmium compounds

Common oxidation states of rhodium is +3, but oxidation states from +0 to +6 are observed.⁵

Unlike ruthenium and osmium, rhodium forms no stable volatile oxygen compounds. Only the Rh₂O₃, RhO₂·xH₂O and the Na₂RhO₃ are stable. Halogen compounds are known in the nearly the full range of possible oxidation states. Rhodium(III) chloride, rhodium(IV) fluoride, rhodium(V) fluoride and rhodium(VI) fluoride are some examples. The lower oxidation states are only stable if ligands are present.

Wilkinson's catalyst

^{citation needed}

The best known example is the Wilkinson's catalyst (chlorotris(triphenylphosphine)rhodium(I). The catalyst is for example used for the hydrogenation of alkenes.⁶

Isotopes

Main article: isotopes of rhodium

Naturally occurring rhodium is composed of only one isotope, ¹⁰³Rh. The most stable radioisotopes are ¹⁰¹Rh with a half-life of 3.3 years, ¹⁰²Rh with a half-life of 207 days, ^102mRh with a half-life of 2.9 years, and ⁹⁹Rh with a half-life of 16.1 days. Twenty other radioisotopes have been characterized with atomic weights ranging from 92.926 u (⁹³Rh) to 116.925 u (¹¹⁷Rh). Most of these have half-lives that are less than an hour except ¹⁰⁰Rh (half-life: 20.8 hours) and ¹⁰⁵Rh (half-life: 35.36 hours). There are also numerous meta states with the most stable being ^102mRh (0.141 MeV) with a half-life of about 207 days and ^101mRh (0.157 MeV) with a half-life of 4.34 days. See isotopes of rhodium.⁷

The primary decay mode before the only stable isotope, ¹⁰³Rh, is electron capture and the primary mode after is beta emission. The primary decay product before ¹⁰³Rh is ruthenium and the primary product after is palladium.^{citation needed}

History

Rhodium (Greek rhodon meaning "rose") was discovered in 1803 by William Hyde Wollaston,⁸ soon after his discovery of palladium.⁹¹⁰ He made this discovery in England using crude platinum ore that he presumably obtained from South America.¹¹

His procedure involved dissolving the ore in aqua regia, neutralizing the acid with sodium hydroxide (NaOH). He then precipitated the platinum by adding ammonium chloride, NH₄Cl, as ammonium chloroplatinate. All other metals like copper, lead, palladium and rhodium were precipitated with zinc. Diluted nitric acid dissolved all but palladium and rhodium,which were dissolved in aqua regia and the rhodium was precipitated by the addition of sodium chloride as Na₃[RhCl₆·nH₂O. After washing with ethanol the rose red precipitate was reacted with zinc forming rhodium metal.¹²

Applications

Rhodium foil and wire

The primary use of this element is in automobiles as a catalytic converter, which converts harmful emissions from the engine into less harmful gases.¹³¹⁴

Catalytic converter

Cross section of a Metal-core Converter

In 2007 81%¹³ of the world production of rhodium were consumed to produce three-way catalytic converters.¹³ Rhodium shows some advantages over the other platin metals in the reduction of nitrogen oxides to nitrogen and oxygen:¹⁵

2NO_x → xO₂ + N₂

The recycling of catalytic converter also became a valuable source for rhodium. In 2007 5.7 t were extracted from this source. Compared to the 22 t which have been mined, this is a relative high recycling rate.¹³

Other uses

Rhodium is used as an alloying agent for hardening and improving the corrosion resistance¹⁶ of platinum and palladium. These alloys are used in furnace windings, bushings for glass fiber production, thermocouple elements, electrodes for aircraft spark plugs, and laboratory crucibles.¹⁷ Other uses include:

• It is 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.

Rhodium plated white gold wedding ring

• This metal finds use in jewelry and for decorations. It is electroplated on white gold and platinum to give it a reflective white surface. This is known as rhodium flashing in the jewelry business. It also may be used in coating sterling silver in order to strengthen the metal from tarnish, as a result from the copper compound found in sterling silver.
• It is also a highly useful catalyst in a number of industrial processes (notably it is used in the catalytic system of automobile catalytic converters and for catalytic carbonylation of methanol to produce acetic acid by the Monsanto process). It is used to catalyse addition of hydrosilanes to a double bond, a process important in manufacture of certain silicone rubbers.
• The complex of a rhodium ion with BINAP gives a widely used chiral catalyst for chiral synthesis, as in the synthesis of menthol.
• It is also used as a filter in mammography systems because of the characteristic x-rays it produces.
• It is also used in high quality pen surfaces due to its high-resistance characteristics. These pens include Graf von Faber-Castell, Caran D'ache.
• Rhodium neutron detectors are used in Combustion Engineering Nuclear Reactors to measure neutron flux levels - a method that requires a digital filter to determine the current neutron flux level, as there are three signals generated: immediate, a few seconds later, and a minute later, each with its own signal level, and all three are combined in the rhodium detector signals. The three Palo Verde nuclear reactors each have 305 rhodium neutron detectors, 61 detectors on each of 5 vertical levels, providing an accurate 3-D "picture" of reactivity, allowing fine tuning to most economically burn the nuclear fuel.

Occurrence

Normal mining

The industrial extraction of rhodium is complex as the metal occurs in ores mixed with other metals such as palladium, silver, platinum, and gold. It is found in platinum ores and obtained free as a white inert metal which is very difficult to fuse. Principal sources of this element are located in South Africa, in river sands of the Ural Mountains, and in North America, including 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 (>80%) followed by Russia.¹⁸ The annual world production of this element is only about 25 tons and there are very few rhodium-bearing minerals. As of October 2007, rhodium cost approximately eight times more than gold, 450 times more than silver, and 27,250 times more than copper by weight. Rhodium's typical historical price is about $1,000/troy oz,¹⁹ but in recent years it has increased to about $4500/troy oz.²⁰ In 2008 the price briefly rose above $10,000 per ounce.^{citation needed} The 3rd quarter 2008 economic slowdown has pushed prices sharply back below $1,000 per ounce, however.^{citation needed}

As a fission product

It is also possible to extract rhodium from used nuclear fuel, which contains rhodium (1 kg of the fission products of ²³⁵U contain 13.3 grams of ¹⁰³Rh). So as a typical used fuel is 3% fission products by weight it will contain about 400 grams of rhodium per ton of used fuel. The longest lived radioisotope of rhodium is ^102mRh which has a half life of 2.9 years, while the ground state (¹⁰²Rh)has a half life of 207 days.

The radioactivity in MBq per gram of each of the platinum group metals which are formed by the fission of uranium, ruthenium is the most radioactive. Palladium has an almost constant activity due to the very long lived ¹⁰⁷Pd, while rhodium is the least radioactive

Each kilo of fission rhodium will contain 6.62 ng of ¹⁰²Rh and 3.68 ng of ^102mRh. As ¹⁰²Rh decays by beta decay to either ¹⁰²Ru (80%) (some positron emission will occur) or ¹⁰²Pd (20%) (some gamma ray photons with about 500 keV are generated) and the excited state decays by beta decay (electron capture) to ¹⁰²Ru (some gamma ray photons with about 1 MeV are generated). If the fission occurs in an instant then 13.3 grams of rhodium will contain 67.1 MBq (1.81 mCi) of ¹⁰²Rh and 10.8 MBq (291 μCi) of ^102mRh. As it is normal to allow used nuclear fuel to stand for about five years before reprocessing, much of this activity will decay away leaving 4.7 MBq of ¹⁰²Rh and 5.0 MBq of ^102mRh. If the rhodium metal was then left for 20 years after fission, then the 13.3 grams of rhodium metal would contain 1.3 kBq of ¹⁰²Rh and 500 kBq of ^102mRh. At first glance the rhodium might be adding to the resource value of reprocessed fission waste, but the cost of the separation of the rhodium from the other metals needs to be considered.

Precautions

Rhodium metal is, as a noble metal, inert. However, when rhodium is chemically bound, it is reactive. Lethal intake (LD₅₀) for rats is 12.6 mg/kg of rhodium chloride (RhCl₃).²¹ Rhodium compounds can strongly stain human skin. The element plays no biological role in humans. If used in elemental form rather than as compounds, the metal is harmless.²²

Ornamental uses

Rhodium has been used for honours, or to symbolize wealth, 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.

See also

• Rhodium compounds

References

External links

• WebElements.com – Rhodium
• Current Rhodium price
• Rhodium Technical and Safety Data
• Los Alamos National Laboratory – Rhodium

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