Noble Metal
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Noble Metal

Noble metals in the periodic table
  Elements categorised as such[1]
  Also recognised by (Arb) Brooks[2]
  Arb Ahmad[3]
  Arb Wells[4]
  Arb Tamboli et al.[5]
  Elements commonly recognised as metalloids
  Noble gases

In chemistry, noble metals are metallic elements that show outstanding resistance to chemical attack even at high temperatures.[6] They are well known for their catalytic properties and associated capacity to facilitate or control the rates of chemical reactions.[6] The short list of chemically noble metals (those elements upon which almost all chemists agree) comprises ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au).[7] In periodic table terms the noble metals correspond to the noble gases.[8]

More inclusive lists include one or more of copper (Cu), silver (Ag), rhenium (Re), and mercury (Hg) as noble metals. On the other hand, titanium (Ti), niobium (Nb), and tantalum (Ta) are not included as noble metals although they are very resistant to corrosion.

A collection of the noble metals, including copper, silver, rhenium and mercury, which are also recognised as such by some authors. They are arranged according to their position in the periodic table.

History and meanings

While the noble metals tend to be valuable - due to both their rarity in the Earth's crust and their applications in areas like metallurgy, high technology, and ornamentation (jewelry, art, sacred objects, etc.) - the terms noble metal and precious metal are not synonymous.

The term noble metal can be traced back to at least the late 14th century[9] and has slightly different meanings in different fields of study and application. Only in atomic physics is there a strict definition, which includes only copper, silver, and gold, because they have completely filled d-subshells. Although noble metal lists can differ, they tend to cluster around the six platinum group metals namely, ruthenium, rhodium, palladium, osmium, iridium, and platinum; plus gold.

In addition to this term's function as a compound noun, there are circumstances where noble is used as an adjective for the noun metal. A galvanic series is a hierarchy of metals (or other electrically conductive materials, including composites and semimetals) that runs from noble to active, and allows one to predict how materials will interact in the environment used to generate the series. In this sense of the word, graphite is more noble than silver and the relative nobility of many materials is highly dependent upon context, as for aluminium and stainless steel in conditions of varying pH.[10]


Abundance of the chemical elements in the Earth's crust as a function of atomic number. The rarest elements (shown in yellow, including the noble metals) are not the heaviest, but are rather the siderophile (iron-loving) elements in the Goldschmidt classification of elements. These have been depleted by being relocated deeper into the Earth's core. Their abundance in meteoroid materials is relatively higher. Tellurium and selenium have been depleted from the crust due to formation of volatile hydrides.


The noble metals are siderophiles (iron-lovers). They tend to sink into the Earth's core because they dissolve readily in iron either as solid solutions or in the molten state. Most siderophile elements have practically no affinity whatsoever for oxygen: indeed, oxides of gold are thermodynamically unstable with respect to the elements.

Melting points of oxides, °C
Copper 1326
Ruthenium d1300
Rhodium d1100
Palladium d750
Silver d200
Rhenium 360
Osmium d500
Iridium d1100
Platinum 450
Gold d150
Mercury d500
Strontium ? 2430
Molybdenum ? 801
Antimony MD 655
Lanthanum ? 2320
Bismuth ? 817
d = decomposes; if there are two figures, the 2nd is for
the hydrated form; ? = not a noble metal; MD = metalloid

Corrosion resistance

Platinum, gold and mercury can be dissolved in aqua regia, a highly concentrated mixture of hydrochloric acid and nitric acid, but iridium cannot. The solubility of silver is limited by the formation of silver chloride precipitate.[11] Palladium and silver are, however, soluble in nitric acid. Ruthenium can be dissolved in aqua regia only when in the presence of oxygen, while rhodium must be in a fine pulverized form. Niobium and tantalum are resistant to all acids, including aqua regia.[8]


According to Smith,[12]

"There is no sharp dividing line [between 'noble metals' and 'base metals'] but perhaps the best defintion of a noble metal is a metal whose oxide is easily decomposed at a temperature below a red heat."
"It follows from this that noble metals...have little attraction for oxygen and are consequently not oxidised or discoloured at moderate temperatures."

Such nobility is mainly associated with the relatively high electronegativity values of the noble metals, resulting in only weakly polar covalent bonding with oxygen.[13] The table lists the melting points of the oxides of the noble metals, and for some of those of the non-noble metals, for the elements in their most stable oxidation states.


In physics, the definition of a noble metal is most strict. It requires that the d-bands of the electronic structure be filled. From this perspective, only copper, silver and gold are noble metals, as all d-like bands are filled and do not cross the Fermi level.[14] However, d-hybridized bands do cross the Fermi level to a small extent. In the case of platinum, two d bands cross the Fermi level, changing its chemical behaviour such that it can function as a catalyst. The difference in reactivity can easily be seen during the preparation of clean metal surfaces in an ultra-high vacuum: surfaces of "physically defined" noble metals (e.g., gold) are easy to clean and keep clean for a long time, while those of platinum or palladium, for example, are covered by carbon monoxide very quickly.[15]


The following table lists standard reduction potential in volts;[16][17]; electronegativity (revised Pauling); and electron affinity values (kJ/mol), for some metals and metalloids. Metals commonly recognised as noble metals are flagged with a ✣ symbol; elements marked  ☢ are synthetic, radioactive, and short-lived; metalloids are denoted MD; values with a + are predicted; a blank = not available or applicable.

The simplified entries in the reaction column can be read in detail from the Pourbaix diagrams of the considered element in water. Noble metals have large positve potentials;[18] elements not in this table have a negative standard potential or are not metals.

Electronegativity is included since it is reckoned to be, "a major driver of metal nobleness and reactivity".[13] Predicted values for nihonium, flerovium, moscovium, and livermorium are given by Karol.[19]

On account of their high electron affinity values,[20] the incorporation of a noble metal in the electrochemical photolysis process, such as platinum and gold, among others, can increase photoactivity.[21]

Element Atomic
Group Period Reaction Poten-
tial (V)
Copernicium ☢ 112 12 7 + 2 e- -> Cn 2.1 +
Roentgenium ☢ 111 11 7 + 3 e- -> Rg 1.9 +
Darmstadtium ☢ 110 10 7 + 2 e- -> Ds 1.7 +
Gold 79 11 6 + 3 e- -> Au 1.5 2.54 223
Platinum 78 10 6 + 2 e- -> Pt 1.2 2.28 205
Iridium 77 9 6 + 3 e- -> Ir 1.16 2.2 151
Astatine ☢ 85 17 6 + e- -> At 1.0 2.2 233
Palladium 46 10 5 + 2 e- -> Pd 0.915 2.2 54
Flerovium ☢ 114 14 7 + 2 e- -> Fl 0.9  2.21  +
Osmium 76 8 6 + 4  + 4 e- -> Os + 2  0.85 2.2 104
Mercury 80 12 6 + 2 e- -> Hg 0.85 2.0 -50
Rhodium 45 9 5 + 3 e- -> Rh 0.8 2.28 110
Meitnerium ☢ 109 9 7 + 3 e- -> Mt 0.8  +
Silver 47 11 5 + e- -> Ag 0.7993 1.93 126
Ruthenium 44 8 5 + 3 e- -> Ru 0.6 2.2 101
Polonium ☢ 84 16 6 + 2 e- -> Po 0.6 2.0 136
Nihonium ☢ 113 13 7 + e- -> Nh 0.6 + 2.09  +
TelluriumMD 52 16 5 + 4  + 4 e- -> Te + 2  0.53 2.1 190
Rhenium 75 7 6 + 3 e- -> Re 0.5 1.9 6
Water 75 7 6 + 4 e- + -> 4 OH- 0.4
Hassium ☢ 108 8 7 + 4 e- -> Hs 0.4 +
Copper 29 11 4 + 2 e- -> Cu 0.339 2.0 119
Bismuth 83 15 6 + 3 e- -> Bi 0.308 2.02 91
Technetium ☢ 43 7 5 3 e- -> Tc 0.3 1.9 53
ArsenicMD 33 15 4 + 12  + 12 e- -> 4 As + 6  0.24 2.18 78
AntimonyMD 51 15 5 + 6  + 6 e- -> 2 Sb + 3  0.147 2.05 101
Bohrium ☢ 107 7 7 + 5 e- -> Bh 0.1 +
Livermorium ☢ 116 16 7 + 2 e- -> Lv 0.1 2.58 +

Arsenic, antimony and tellurium are considered to be metalloids rather than noble metals.

The black tarnish commonly seen on silver arises from its sensitivity to hydrogen sulfide: 2Ag + H2S + ½O2 -> Ag2S + H2O. Rayner-Canham[22] contends that, "silver is so much more chemically-reactive and has such a different chemistry, that it should not be considered as a 'noble metal'." In dentistry, silver is not regarded as a noble metal due to its tendency to corrode in the oral environment.[23]

The relevance of the entry for water is addressed by Li et. al.[24] in the context of galvanic corrosion. Such a process will only occur when:

"(1) two metals which have different electrochemical potentials are...connected, (2) an aqueous phases with electrolyte exists, and (3) one of the two metals has...potential lower than the potential of the reaction ( + 4e + = 4 OHo) which is 0.4 V...The...metal with...a potential less than 0.4 V acts as an anode...loses electrons...and dissolves in the aqueous medium. The noble metal (with higher electrochemical potential) acts as a cathode and, under many conditions, the reaction on this electrode is generally - 4 eo - = 4 OHo)."

The superheavy elements from hassium (element 108) to livermorium (116) inclusive are expected to be "partially very noble metals"; chemical investigations of hassium has established that it behaves like its lighter congener osmium, and preliminary investigations of nihonium and flerovium have suggested but not definitively established noble behavior.[25]Copernicium's behaviour seems to partly resemble both its lighter congener mercury and the noble gas radon.[26]

See also


  1. ^ Van Loon, JC (1977). "Analytical chemistry of the noble metals". Pure & Applied Chemistry. 49 (10): 1495-1505. doi:10.1351/pac197749101495.
  2. ^ Brooks, RR (1992). Noble metals and biological systems: Their role in medicine, mineral exploration, and the environment. Boca Raton: CRC Press. p. 1. ISBN 978-0849361647.
  3. ^ Ahmad, Z (2006). Principles of corrosion engineering and corrosion control. Amsterdam: Elsevier. p. 40. ISBN 9780080480336.
  4. ^ Wells, DA (1860). Principles and applications of Chemistry. New York: Ivison, Phinney & Company. p. 885.
  5. ^ Tamboli, D; Osso, O; McEvoy, T; Vega, L; Rao, M; Banerjee, G (2010). "Investigating the compatibility of ruthenium liners with copper interconnects". ECS Transactions. 33 (10). doi:10.1149/1.3489059.
  6. ^ a b Hämäläinen, J; Ritala, M; Leskelä, M (2013). "Atomic layer deposition of noble metals and their oxides". Chemistry of Materials,. 26 (1): 786-801. doi:10.1021/cm402221y.CS1 maint: extra punctuation (link)
  7. ^ A. Holleman, N. Wiberg, "Lehrbuch der Anorganischen Chemie", de Gruyter, 1985, 33. edition, p. 1486
  8. ^ a b A. Holleman, N. Wiberg, "Inorganic Chemistry", Academic Press, 2001
  9. ^ "the definition of noble metal". Retrieved 2018.
  10. ^ Everett Collier, "The Boatowner's Guide to Corrosion", International Marine Publishing, 2001, p. 21
  11. ^ W. Xing, M. Lee, Geosys. Eng. 20, 216, 2017
  12. ^ Smith, JC (1946). The chemistry and metallurgy of dental materials. Oxford: Blackwell. p. 40.
  13. ^ a b Kepp, K (2020). "Chemical causes of metal nobleness". ChemPhysChem. 21 (5): 360-369. doi:10.1002/cphc.202000013.
  14. ^ Hüger, E.; Osuch, K. (2005). "Making a noble metal of Pd". EPL. 71 (2): 276. Bibcode:2005EL.....71..276H. doi:10.1209/epl/i2005-10075-5.
  15. ^ S. Fuchs, T.Hahn, H.G. Lintz, "The oxidation of carbon monoxide by oxygen over platinum, palladium and rhodium catalysts from 10-10 to 1 bar", Chemical engineering and processing, 1994, V 33(5), pp. 363-369 [1]
  16. ^ G. Wulfsberg, "Inorganic Chemistry", University Science Books, 2000, pp. 247-249 ? Bratsch S. G., "Standard Electrode Potentials and Temperature Coefficients in Water at 298.15 K", Journal of Physical Chemical Reference Data, vol. 18, no. 1, 1989, pp. 1-21 ? B. Douglas, D. McDaniel, J. Alexander, "Concepts and Models of Inorganic Chemistry", John Wiley & Sons, 1994, p. E-3
  17. ^ 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.
  18. ^ Ahmad, Z (2006). Principles of corrosion engineering and corrosion control. Amsterdam: Elsevier. p. 40. ISBN 9780080480336.
  19. ^ Karol, PJ (2020). "Extending electronegativities to superheavy Main Group atoms". Chemistry International. 42 (3): 12-15. doi:10.1515/ci-2020-0305.
  20. ^ Viswanathan, B (2002). Catalysis: Principles and Applications. Boca Raton: CRC Press. p. 291.
  21. ^ Fujishima, A.; Honda, K. (1972). "Electrochemical Photolysis of Water at a Semiconductor Electrode". Nature. 238 (5358): 37-38. Bibcode:1972Natur.238...37F. doi:10.1038/238037a0. PMID 12635268. S2CID 4251015.; Nozik, A.J. (1977). "Photochemical Diodes". Appl Phys Lett. 30 (11): 567-570. Bibcode:1977ApPhL..30..567N. doi:10.1063/1.89262.
  22. ^ Rayner-Canham, G (2018). "Organizing the transition metals". In Scerri, E; Restrepo, G (eds.). Mendeleev to Oganesson: A multidisciplinary perspective on the periodic table. Oxford University. pp. 195-205. ISBN 978-0-190-668532.
  23. ^ Powers, JM; Wataha, JE (2013). ? Dental materials: Properties and manipulation? (10th ed.). St Louis: Elsevier Health Sciences. p. 134. ISBN 9780323291507.
  24. ^ Li, Y; Lu, D; Wong, CP (2010). Electrical conductive adhesives with nanotechnologies. New York: Springer. p. 179. ISBN 978-0-387-88782-1.
  25. ^ Nagame, Yuichiro; Kratz, Jens Volker; Matthias, Schädel (December 2015). "Chemical studies of elements with Z >= 104 in liquid phase". Nuclear Physics A. 944: 614-639. Bibcode:2015NuPhA.944..614N. doi:10.1016/j.nuclphysa.2015.07.013.
  26. ^ Mewes, J.-M.; Smits, O. R.; Kresse, G.; Schwerdtfeger, P. (2019). "Copernicium is a Relativistic Noble Liquid". Angewandte Chemie International Edition. doi:10.1002/anie.201906966. Cite has empty unknown parameter: |1= (help)

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