Rhodium

Rhodium, symbol Rh, is the rarest of all non-radioactive metals on Earth, and therefore an expensive metal. On the 22nd July 2011, 1 gram of rhodium cost $(AUD)38 compared to 1 gram of gold which cost $(AUD)31 or 1g of silver for only 78 cents !
Rhodium is a transition metal with a density of 12.41 gcm^-3 and is found in nature as the free metal, or alloyed with similar metals such as platinum or nickel, but not as a chemical compound.
Naturally occurring rhodium is composed of only one isotope, rhodium-103.
Only about 3 tonnes of rhodium are produced in the world each year, and most of this is used for catalyzing chemical reactions.
Approximately 80% of the rhodium produced is used as a reduction catalyst in the three-way catalytic converters of cars.
In a three-way catalytic converter three processes occur simultaneously:

1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NO_x → xO₂ + N₂
2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O₂ → 2CO₂
3. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water

Other uses of rhodium include :

• plating white gold to make it appear more silvery (white gold is actually an alloy of gold with atleast one other metal such as nickel, manganese, palladium)
• plating sterling silver to make it appear more silvery (sterling silver is an alloy of silver containing 92.5% by mass silver and 7.5% by mass of other metals such as copper)

Queen's University chemists have just discovered that rhodium that is modified using carbon, nitrogen or hydrogen-based complexes changes colour to yellow in the presence of nitrogen, deep blue in the presence of oxygen, and brown in the presence of carbon monoxide. Modified metals, such as modified rhodium, that change colour in the presence of particular gases could warn consumers if packaged food has been exposed to air or if there's a carbon monoxide leak at home. This finding could potentially influence the production of both industrial and commercial air quality sensors.

Reference
Queen's University (2011, July 21). Modified metals change color in the presence of particular gases. ScienceDaily. Retrieved July 23, 2011, from http://www.sciencedaily.com­ /releases/2011/07/110721131159.htm

Further Reading
Periodic Table
Definitions of a Mole
Mass-Mole Calculations
Density
Isotopes
Relative Atomic Mass
Metals & Non-metals
Percentage Composition

Study Questions

1. Locate rhodium in the Periodic Table and give its
   • atomic number
   • relative atomic mass
2. Using the prices per gram of metal given in the story above, calculate
   • the price of the 3 tonnes of rhodium produced in the world each year
     
   • the price of 1 mole of rhodium
   • the price of 10 cubic centimeters of rhodium
   • the mass of $57,000 worth of rhodium
   • the volume of $57,000 worth of rhodium
3. Naturally occurring rhodium has only 1 isotope, rhodium-103. For this isotope give:
   • the number of protons in the nucleus of a rhodium atom
   • the number of neutrons in the nucleus of a rhodium atom
   • the mass number of this isotope of rhodium
   • the atomic number for this isotope of rhodium
4. If naturally occurring rhodium only has 1 isotope why is its relative atomic mass 102.9?
5. List the physical properties you would expect rhodium to have based on its position within the Periodic Table.
6. Why would coating white gold in rhodium make it appear more silvery?
7. A 25 kg sample of sterling silver contains only silver and copper.
   • What mass of silver is present in the sample?
   • What mass of copper is present in the sample?
8. A sample of white gold is found to contain only 1.39 g gold and 0.09g of nickel. Calculate the percent by mass of each element present in the sample.