Rhodium is a chemical element with the atomic number 45 in the periodic table. It’s an extremely rare metal that is found in Earth’s crust with an abundance of 0.0002 parts per million.
Being a member of the platinum group of periodic table elements, this noble metal has three valence electrons and is highly appreciated for its corrosion and oxidation resistance, bright lustrous shine, as well as for its durability as a material.
Chemical and Physical Properties of Rhodium
|Atomic weight||102.9055 g.mol-1|
|Color||A precious silver-white metal|
|Physical state||Solid at room temperature|
|Density||12.4 g.cm-3 at 20°C|
|Melting point||1963°C, 3565°F, 2236 K|
|Boiling point||3695°C, 6683°F, 3968 K|
|Ionic radius||75 (3+) pm|
|Most characteristic isotope||rhodium-103|
|Electronic shell||[Kr] 4d8 5s1|
|The energy of the first ionization||742 kJ.mol-1|
|Uses||Used as a coating to prevent wear on high-quality science equipment and with platinum to make thermocouples.|
|Description||Soft, malleable, ductile, silvery-white metal.|
|Crystal Structure||Cubic: Face centered|
|Covalent Radius||1.25 Å|
|Atomic Radius||1.83 Å|
|Atomic Volume||08.3 cm³/mol|
|Name Origin||Greek: rhodon (rose). Its salts give a rosy solution.|
With the periodic table symbol Rh, atomic number 45, atomic mass of 102.91g.mol-1, and electron configuration [Kr] 4d8 5s1, rhodium is a hard, silvery-white precious metal that does not tarnish upon exposure to air.
It reaches its boiling point at 3695°C, 6683°F, 3968 K, while the melting point is achieved at 1963°C, 3565°F, 2236 K. This member of the platinum family of elements in the periodic table has an electronegativity of 2.2 according to Pauling, whereas the atomic radius according to van der Waals is unknown.
Classified as a transition metal, rhodium has strong anti-corrosive properties and has the capacity to resist extremely high temperatures. It’s also characterized by a lower density and a higher melting point than platinum.
How Was Rhodium Discovered?
Element 45 was discovered in 1804 by the English chemist and physicist William Hyde Wollaston (1766 – 1828). After the French chemist Hippolyte-Victor Collet-Descotils expressed his belief that a sample of platinum ore obtained from South Africa could possibly contain a new metal, he supported his claim by pointing to the presence of the red color of some platinum salts.
This triggered Wollason’s scientific curiosity, so he took a closer look at the platinum sample. First, the English chemist dissolved crude platinum in aqua regia, after which he precipitated platinum metal by dissolving the solution in ammonium chloride.
The remaining liquid substance was furthermore included in several other chemical reactions, after which Wollaston succeeded in producing a deep red powder, i.e sodium rhodium chloride. In the last step of this series of chemical reactions, the rhodium precipitate was treated with zinc. Finally, the presence of the new element 45 in the platinum ore was evident and confirmed.
How Did Rhodium Get Its Name?
The name of this chemical element comes from the Greek word ‘rhodon‘, meaning ‘rose colored’. This refers to the characteristic rose-red color of the rhodium salts.
Where Can You Find Rhodium?
As a member of the platinum group, rhodium mostly occurs in the same mineral ores as the other platinum metals, such as platinum, palladium, nickel, silver, and gold. It can be isolated from the minerals bowieite, rhodite, and rhodplumsite, in which rhodium occurs as a part of a compound.
Primary rhodium mines are rarely found. Thus, this rare metal is typically obtained as a by-product of platinum and palladium mining or nickel mining. It can also be traced in the native alloys of iridium (at least 11.25 percent) and osmium in iridosmine (at least 4.5 percent in siserskite).
For commercial purposes, element 45 is mostly isolated as a by-product of the extraction of nickel and copper from their ores. South Africa, South America, Canada, and Russia, are the world’s leading rhodium producers. The price of rhodium is typically high, but also highly variable on the precious metals market.
Rhodium in Everyday Life
Rhodium is a noble metal praised for its superb anti-corrosive properties, as well as for its thermal and oxidation resistance. The practical uses of this rare chemical with a high price can be observed in the following instances:
- Car industries use rhodium with internal combustion engines fueled by either gasoline or diesel for reduction of the toxic exhaust gases;
- Allows made with rhodium are used in the manufacturing of high-temperature thermocouple and resistance wires, pen nibs, phonograph needles, electrodes for aircraft spark plugs, bearings, and electrical contacts;
- Rhodium is commonly electroplated onto metal objects; when it’s polished, it gives high shine to jewelry. Similarly, rhodium metal is used metal for creating reflecting surfaces for optical instruments;
- When added to platinum, rhodium adds both hardness and lightness to the alloys which are further used in the production of laboratory furnace crucibles, as well as catalysts in very hot chemical environments (including automobile catalytic converters), thermocouple elements, and headlight reflectors. Rhodium’s presence in cars supports the transformation of harmful unburned hydrocarbons, carbon monoxide, and nitrogen oxide exhaust emissions into less noxious gases;
- The chemical industry also uses this noble metal in the production of nitric acid, acetic acid, and hydrogenation reactions;
- The rhodium-platinum alloy is used in the production of heart pacemakers;
- Rhodium plating of white gold gives the shiny silvery-white finish to the jewelry;
- Rhodium’s resistance to tarnishing and its bright shine are employed in the making of searchlights and mirrors.
How Dangerous Is Rhodium?
The rhodium salts are extremely toxic substances, while rhodium is suspected to be a carcinogen element. On the other hand, rhodium metal is a confirmed hypoallergenic, which makes it suitable to be used as a protective coating of white gold jewelry with some allergy-triggering nickel in it.
Environmental Effects of Rhodium
Since rhodium is one of the rarest elements found in the earth’s crust, the environmental effects of this chemical cannot be considered. However, the improper waste disposal of the industries that include rhodium in their production could potentially pollute the environment.
Isotopes of Rhodium
There are 33 isotopes of rhodium observed, with atomic mass ranging from rhodium-89 to rhodium-122. Naturally occurring rhodium consists of only one stable isotope – 103Rh. The primary decay mode before this stable form of element 45 is electron capture, while the primary decay mode after it is beta emission.
|Z||N||Isotopic mass (Da)
[n 2][n 3]
[n 7][n 4]
|Natural abundance (mole fraction)|
|Excitation energy[n 4]||Normal proportion||Range of variation|
|94Rh||45||49||93.92170(48)#||70.6(6) s||β+ (98.2%)||94Ru||(2+, 4+)|
|β+, p (1.79%)||93Tc|
|102Rh||45||57||101.906843(5)||207.0(15) d||β+ (80%)||102Ru||(1−, 2−)|
|104Rh||45||59||103.906656(3)||42.3(4) s||β− (99.55%)||104Pd||1+|
|105Rh[n 8]||45||60||104.905694(4)||35.36(6) h||β−||105Pd||7/2+|
|114Rh||45||69||113.91881(12)||1.85(5) s||β− (>99.9%)||114Pd||1+|
|β−, n (<.1%)||113Pd|
|116Rh||45||71||115.92406(15)||0.68(6) s||β− (>99.9%)||116Pd||1+|
|β−, n (<.1%)||115Pd|
List of Rhodium Compounds
In compounds, rhodium mostly adopts the +1 and +3 oxidation states. It’s not affected by acids – rhodium cannot be dissolved by hot concentrated nitric or hydrochloric acids, or even by aqua regia. Rhodium does not react with oxygen either, which makes this chemical resistant to tarnishing in the air.
The list of most commonly prepared rhodium compounds includes:
- Dichlorotetrakis(pyridine)rhodium(III) chloride
- Pentaamminechlororhodium dichloride
- Rhodium acetylacetonate
- Rhodium hexafluoride
- Rhodium pentafluoride
- Rhodium trifluoride
- Rhodium-platinum oxide
- Rhodium(II) acetate
- Rhodium(III) bromide
- Rhodium(III) chloride
- Rhodium(III) iodide
- Rhodium(III) nitrate
- Rhodium(III) oxide
- Rhodium(III) perchlorate
- Rhodium(III) sulfate
- Rhodium(III) sulfide
- Rhodium(IV) fluoride
- Rhodium(IV) oxide
- Tris(triphenylphosphine)rhodium carbonyl hydride
- Uranium rhodium germanium
- Wilkinson’s catalyst
- Xenon hexafluororhodate
- Ytterbium dirhodium disilicide
5 Interesting Facts and Explanations
- William Wollaston discovered not one, but two new elements in the same year – the chemicals rhodium and palladium.
- In 1976, Volvo introduced rhodium as a catalytic converter in the automobile fuel industry. Since then, this chemical element is widely used as an agent that reduces toxic gases and pollutants in exhaust fumes.
- The platinum group metals (PGM) incorporate the elements platinum, palladium, rhodium, osmium, iridium, and ruthenium.
- Platinum-group elements are referred to as precious metals, because of their stable electron configuration and strong corrosion and oxidation resistance. They are also very durable and resistant materials, which makes them even more valuable.
- Rhodium is considered one of the rarest substances that have ever been detected on our planet.