Noble metal

Periodic table extract showing approximately how often each element tends to be recognized as a noble metal:
 7  most often (Ru, Rh, Pd, Os, Ir, Pt, Au)[1]  1  often (Ag)[2]  2  sometimes (Cu, Hg)[3]  6  in a limited sense (Tc, Re, As, Sb, Bi, Po)
The thick black line encloses the seven to eight metals most often to often so recognized. Silver is sometimes not recognized as a noble metal on account of its greater reactivity.[4]
* may be tarnished in moist air or corrode in an acidic solution containing oxygen and an oxidant
† attacked by sulfur or hydrogen sulfide
§ self-attacked by radiation-generated ozone

A noble metal is ordinarily regarded as a metallic element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals (ruthenium, rhodium, palladium, osmium, iridium) are most often so classified. Silver, copper, and mercury are sometimes included as noble metals, but each of these usually occurs in nature combined with sulfur.

In more specialized fields of study and applications the number of elements counted as noble metals can be smaller or larger. It is sometimes used for the three metals copper, silver, and gold which have filled d-bands, while it is often used mainly for silver and gold when discussing surface-enhanced Raman spectroscopy involving metal nanoparticles. It is sometimes applied more broadly to any metallic or semimetallic element that does not react with a weak acid and give off hydrogen gas in the process. This broader set includes copper, mercury, technetium, rhenium, arsenic, antimony, bismuth, polonium, gold, the six platinum group metals, and silver.

Many of the noble metals are used in alloys for jewelry or coinage. In dentistry, silver is not always considered a noble metal because it is subject to corrosion when present in the mouth. All the metals are important heterogeneous catalysts.

  1. ^ Balcerzak, M (2021). "Noble Metals, Analytical Chemistry of". Encyclopedia of Analytical Chemistry: Applications, Theory and Instrumentation. Wiley Online Library. pp. 1–36. doi:10.1002/9780470027318.a2411.pub3. ISBN 9780471976707.
  2. ^ Schlamp, G (2018). "Noble metals and noble metal alloys". In Warlimont, H; Martienssen, W (eds.). Springer Handbook of Materials Data. Springer Handbooks. Cham: Springer. pp. 339–412. doi:10.1007/978-3-319-69743-7_14. ISBN 978-3-319-69741-3.
  3. ^ Kepp, KP (2020). "Chemical causes of nobility" (PDF). ChemPhysChem. 21 (5): 360–369. doi:10.1002/cphc.202000013. PMID 31912974. S2CID 210087180.
  4. ^ Cite error: The named reference RC was invoked but never defined (see the help page).