Rhodium Facts – Rh or Atomic Number 45


Rhodium Facts

Rhodium is a rare, silvery-white metallic element that belongs to the platinum group metals (PGMs) and also the noble metals. Known for its exceptional reflective properties and high resistance to corrosion, rhodium plays a crucial role in various industrial applications. Its chemical symbol is Rh, and it is atomic number 45 on the periodic table.

Discovery, Naming, and Isolation

Rhodium was discovered in 1803 by English chemist William Hyde Wollaston, who isolated the element from crude platinum ore obtained from South America. The separation from platinum stemmed from the fact that platinum dissolved in aqua regia, but rhodium did not. Wollaston named the element “rhodium” after the Greek word “rhodon,” meaning rose, due to the rose-colored appearance of its salts.

Appearance and Properties

Rhodium exists in a single metallic form with a face-centered cubic (fcc) crystal structure. It has a lustrous, silvery-white appearance and high reflectivity. Rhodium is one of the most corrosion-resistant metals and does not tarnish in air. It is a hard metal with a high melting point and excellent electrical conductivity. Rhodium has a higher melting point and lower density than platinum.

Electron Levels of a Rhodium Atom

Element Group

Rhodium is part of the transition metals, platinum group metals, and is in group 9 of the periodic table. It shares chemical similarities with other members of the group, such as platinum and palladium, and exhibits notable catalytic properties and high corrosion resistance.

Rhodium Isotopes

In nature, rhodium occurs as a single stable isotope, Rh-103. The most notable synthetic isotopes include Rh-101, Rh-102, and Rh-104, which are used in various scientific research applications. Rh-101 is the most stable radioisotope, with a half-life of 3.3 years.

Abundance and Sources

Rhodium is one of the rarest elements in the Earth’s crust, with an estimated abundance of 1 part per 200 million. While it sometimes occurs as a relatively pure native element, it typically comes from platinum or nickel ores. Some nickel meteorites contain the element at levels around 1 part per billion. Potatoes are notably high in rhodium, with concentrations of approximately 0.8 and 30 part per trillion. The mining operations in South Africa produce the majority of the world’s rhodium. Other significant sources include Russia and Canada.

Purification

The purification of rhodium involves a series of complex processes. The first step is flotation and smelting of the ore. The resulting matte then undergoes a series of chemical treatments to remove impurities and isolate the rhodium. This often includes processes such as solvent extraction, precipitation, and finally, reduction to obtain pure rhodium metal.

Uses of Rhodium

Rhodium has several important industrial applications due to its unique properties:

  • Catalysis: Rhodium is a key component in catalytic converters, which reduce harmful emissions from automobile exhausts. Around 80% of the world’s production of the element goes to this use.
  • Electroplating: Rhodium’s reflective properties make it ideal for electroplating jewelry, mirrors, and searchlights. White gold and sterling silver commonly have a thin rhodium coating for tarnish resistance and enhancing shine. Automobile headlight reflectors often use rhodium.
  • Electrical Contacts: Due to its high corrosion resistance, rhodium is useful in electrical contacts and connectors.
  • Alloys: Alloying platinum and palladium with rhodium improves hardness and resistance to oxidation.
  • Chemical Industry: It is a catalyst in various chemical reactions, including the hydrogenation of unsaturated hydrocarbons and in the production of acetic acid. A rhodium complex plays a role in curing certain two-part silicones.
  • Nuclear Reactors: Rhodium finds use in detectors for measuring the neutron flux level of reactors.
  • X-Rays: Mammography x-rays use rhodium filters for improving imaging.

Oxidation States

Rhodium exhibits a range of oxidation states from -3 to +6. The most common oxidation state is +3, followed by the +1 state. Rhodium(III) compounds, such as rhodium chloride (RhCl3), are widely studied. Rhodium(I) complexes are also significant, especially in homogeneous catalysis.

Biological Role, Health Effects, and Toxicity

Like the other noble metals, rhodium has no known biological role in humans or other organisms. It is not an essential element for life. In terms of toxicity, rhodium metal and its compounds are generally considered to have low toxicity. However, exposure to powdered metal or rhodium compounds causes skin irritation and respiratory issues.

Key Rhodium Facts for Scientists

PropertyValue
NameRhodium
SymbolRh
Atomic Number45
Atomic Weight102.905
Group9
Period5
Blockd-block
Electron Configuration[Kr] 4d8 5s1
Electrons per Shell2, 8, 18, 16, 1
State at Room TempSolid
Melting Point1964 °C (3567 °F)
Boiling Point3695 °C (6683 °F)
Density12.42 g/cm³
Heat of Fusion26.59 kJ/mol
Heat of Vaporization493 kJ/mol
Molar Heat Capacity24.98 J/(mol·K)
Oxidation States-3, -1, 0, +1, +2, +3, +4, +5, +6
Electronegativity2.28
Ionization Energies1st: 719.7 kJ/mol
2nd: 1740 kJ/mol
3rd: 2997 kJ/mol
Atomic Radius134 pm
Covalent Radius142 pm
Crystal StructureFace-centered cubic (FCC)
Thermal Conductivity150 W/(m·K)
Electrical Resistivity43.3 nΩ·m
Magnetic OrderingParamagnetic
Young’s Modulus380 GPa
Shear Modulus150 GPa
Mohs Hardness6

References

  • Heck, R.; Farrauto, Robert J. (2001). “Automobile exhaust catalysts”. Applied Catalysis A: General. 221 (1–2): 443–457. doi:10.1016/S0926-860X(01)00818-3
  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  • Landolt, Robert R.; Berk Harold W.; Russell, Henry T. (1972). “Studies on the toxicity of rhodium trichloride in rats and rabbits”. Toxicology and Applied Pharmacology. 21 (4): 589–590. doi:10.1016/0041-008X(72)90016-6
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. ISBN 0-8493-0464-4.
  • Wollaston, W. H. (1804). “On a New Metal, Found in Crude Platina”. Philosophical Transactions of the Royal Society of London. 94: 419–430. doi:10.1098/rstl.1804.0019