What Are Noble Metals? – Definition and List

In chemistry, noble metals are metallic elements that resist oxidation, even at high temperatures. The term “noble metal” dates back to at least the late 14th century, describing metallic elements that are fairly unreactive to oxygen, much like the noble gases are nearly inert nonmetals. The opposite of a noble metal is a base metal.

But, the definition of noble metals and list of elements included in the group varies somewhat between disciplines. For example, in physics, a noble metal is a metallic element with filled electron d-bands.

List of Noble Metals

The noble metals are the six platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, platinum), plus silver and gold. Some chemists include rhenium and mercury. Others also include copper.

Under the physics definition, the only noble metals are copper, silver, and gold.

Noble Metal Properties

The noble metals share several common properties:

• Resist oxidation: Resistance to oxidation is the defining characteristic of a noble metal. These elements can form oxides. For example, silver tarnishes and copper oxidizes to form verdigris. However, noble metal oxides readily decompose when exposed to heat. Similarly, noble metals resist oxidation in moist air and hot water.
• Resist corrosion: Noble metals resist attack by acids and other chemicals, but the level of resistance varies according to the element. For example, palladium and silver dissolve in nitric acid, but platinum and gold resist acids except for aqua regia. Some metals that resist corrosion are not noble metals, such as titanium, niobium, and tantalum.
• High electrical conductivity: In general, metals are good conductors of heat and electricity. But, the noble metals are among the best electrical conductors. Their corrosion resistance makes them popular choices for electrodes, contacts, and wires.
• Catalytic activity: The partially filled d-subshells of noble metals (under the chemistry definition) make these elements excellent catalysts.
• Electron affinity: The noble metals have high electron affinity values.
• Siderophilic: The noble metals are siderophiles (“iron lovers”). They readily dissolve in molten iron or iron solutions. As a consequence, these elements likely accumulate in the Earth’s core.
• Native elements: The six platinum group metals, copper, silver, and gold are the only metals that occur in relatively pure form in nature (native).

Noble Metal Uses

Noble metals find use in coinage, jewelry, medicine, electronics, as protective coatings, and in chemistry as catalysts. Platinum, gold, silver, and palladium are bullion metals that are used in coins and jewelry. Noble metals are often plated over base metals to protect them and add value. Copper, gold, and silver are used in medicine as antimicrobial agents. Copper, gold, and silver are used in wires, contacts, and electrodes. Platinum, palladium, rhodium, ruthenium, and iridium are important catalysts. Ruthenium and iridium are hard metals that strengthen alloys and find use in small machine parts, pen nibs, and spark plugs.

References

• American Geological Institute (1997). Dictionary of Mining, Mineral, and Related Terms (2nd ed.).
• Brooks, Robert R., ed. (1992). Noble Metals and Biological Systems: Their Role in Medicine, Mineral Exploration, and the Environment. Boca Raton, FL.: CRC Press.
• Hoffman, Darleane C.; Lee, Diana M.; Pershina, Valeria (2006). “Transactinides and the future elements.” In Morss; Edelstein, Norman M.; Fuger, Jean (eds.). The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Dordrecht, The Netherlands: Springer Science+Business Media. ISBN 1-4020-3555-1.
• Hüger, E.; Osuch, K. (2005). “Making a noble metal of Pd.” EPL. 71 (2): 276. doi:10.1209/epl/i2005-10075-5