The Robust Element Rhodium


The Element Rhodium

Rhodium is an extremely rare element. It’s silvery-white, corrosion resistant, and a chemically inert transition metal. Rhodium’s atomic symbol is derived from the Greek word “rhodon”, which translates to ‘rose coloured’.

Seven Interesting Facts About Rhodium

  1. The first-ever encountered compound of rhodium was a beautiful deep red color
  2. Rhodium is the rarest of all non-radioactive metals, and therefore can be very expensive
  3. Surprisingly, Rhodium is insoluble in most acids
  4. South-Africa, Russia and Zimbabwe are amongst the top-producers of rhodium
  5. Rhodium has no-known biological role
  6. From 2020 to 2021, the price of rhodium tripled, from $10,000 an ounce to $30,000 an ounce, and then fell back down to $10,000 an ounce.
  7. Rhodium is considered the most expensive non-radioactive element

Rhodium in the Periodic Table

Rhodium has the symbol Rh and the atomic number 45. The element rhodium has 6 valence electrons. It has an electronegativity of 2.28.

Rhodium is in group 9 and row 5. Therefore, this placement makes it part of the cobalt family. It is a transition metal and part of the d-block elements. Additionally, above rhodium on the periodic table is cobalt and below it is iridium.

Unique Properties of Rhodium

Rhodium is an extremely rare element, with a high-melting point, and low resistance. It’s properties make it ideal for use in manufacturing laboratory crucibles and catalyst-gauzes. Another property of rhodium, is the silvery-white appearance and highly-reflective of light makes it optimal for plating jewelry and coating mirrors too.

Rhodium’s Applications in Today’s World

Uses for Rhodium

Rhodium in Alloys

Rhodium produces hardened material when alloyed with other metals such as iridium and platinum. Generally, this type of material is ideal for coating laboratory crucibles, optic mirrors, and furnace windings, and x-ray filters (for breast cancer diagnosis).

Rhodium in Catalytic Converters

Rhodium is primarily used in the production of an automobile device called a catalytic converter. A catalytic converter in an automobile reduces the toxicity of the gases which the vehicle releases. Such a converter – which is made of platinum, rhodium, and other substances – uses reduction and oxidization processes to make pollutant gases less noxious.

To better understand this concept, visualize this situation:

A vehicle is about to release a nitrogen oxide molecule. When it passes the catalytic converter it will undergo a redox-reaction. In this reaction, rhodium (and platinum) will retain the nitrogen molecule while releasing the oxygen molecule. Thus, the toxicity of the pollutant (nitrogen oxide), has been reduced. 

Reaction diagram of a toxic molecule that is prevented by rhodium element
The diagram above is a representation of a potential reaction happening within a catalytic converter and the final result

Rhodium As A Catalyst

Rhodium is used as a catalyst in several industries. The BP-Monsanto process (which is a method of creating acetic acid), is an excellent example of rhodium’s catalyzing capabilities. Rhodium can also be alloyed with platinum to produce a gauze-catalyst which is used for the manufacturing of nitric acid.

When and How was Rhodium Discovered?

In 1803, William Wallostan and Smithson Tennant were attempting to produce pure platinum for commercial purposes. The first step in producing this pure platinum was to dissolve ordinary platinum into an acid mixture called aqua regia. This mixture was to dissolve the ordinary platinum and then retain the pure metal in the form of a precipitate, but when Wollaston tried using the aqua regia, he noticed that he couldn’t dissolve all of the ordinary platinum in it – and much of it remained as a black residue. From this residue, Tennant was able to isolate a few platinum group-metals (PGM) such as iridium, osmium, and palladium. From there, remained a solution called sodium rhodium chloride (Na3RhCl₆), and it contained bright red-crystals. It was through this deep-red solution of sodium rhodium chloride, that Wollaston was able to obtain a sample of rhodium from.

Rhodium – Compounds, Reactions, Isolation, and Oxidization States

Isolation of Rhodium

Rhodium is found in nature in the elemental state in tiny quantities, usually with platinum. The extraction process is complicated, you can read about it here and here.

Rhodium Compounds

A few examples of rhodium compounds include sodium rhodium chloride (Na3RhCl₆), dirhodium tetraacetate Rh2(O2CCH3)4 , and rhodium trichloride RhCl3.

Chemical Properties of Rhodium

Metallic rhodium will react with fluorine gas to produce rhodium (VI) fluoride. Rhodium is inert to most acids and does not react with water (under normal conditions).

Rhodium Oxidization States

Rhodium’s common oxidization state is +3. Other oxidization states include: +1, +2, +3, +4, +5, +6.

Rhodium Properties

  • Symbol: Rh
  • Melting Point: 1963°C, 3565°F, 2236 K 
  • Boiling Point: 3695°C, 6683°F, 3968 K 
  • Density: 12.4 (g cm−3)
  • Relative Atomic Mass: 102.906  
  • Atomic Number: 45
  • Electronegativity: 2.28 (Pauling Scale) 
  • Classification: Platinum Group Metal (PGM) 
  • Crustal Abundance: 0.000037 ppm 
  • Electron Shell Configuration: [Kr] 4d85s1
  • Key Isotopes: 103Rh 

Where Can I Buy Rhodium?

Jewelery stores usually sell rhodium. But it’s quite expensive to purchase in bulk quantities. As of October 2021, the price was around $330 per gram.

Enjoyed learning about Rhodium? Check out other elements on our Interactive Periodic Table!