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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.

Meaning and history

While lists of noble metals can differ, they tend to cluster around gold and the six platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum.

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.

The term noble metal can be traced back to at least the late 14th century and has slightly different meanings in different fields of study and application.

Prior to Mendeleev's publication in 1869 of the first (eventually) widely accepted periodic table, Odling published a table in 1864, in which the "noble metals" rhodium, ruthenium, palladium; and platinum, iridium, and osmium were grouped together, and adjacent to silver and gold.

• Chalcopyrite, which is copper iron sulfide (CuFeS₂), is the most abundant copper ore mineral
• One half of a ruthenium bar.
  Size ~ 40 × 15 × 10 mm
  Weight ~44 g
• Rhodium: 1 g powder, 1g pressed cylinder, 1 g pellet.
• Palladium
• Acanthite, or silver sulfide (Ag₂S)
• Osmium crystals, 2.2 g
• Pieces of pure iridium, 1 g, size: 1–3 mm each
• Crystals of pure platinum
• Gold nugget from Australia, nearly 9,000 g or 317 oz
• Cinnabar or mercury sulfide (HgS) is the most common source ore for refining elemental mercury

Properties

Geochemical

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.

Copper, silver, gold, and the six platinum group metals are the only native metals that occur naturally in relatively large amounts.

Corrosion resistance

Noble metals tend to be resistant to oxidation and other forms of corrosion, and this corrosion resistance is often considered to be a defining characteristic. Some exceptions are described below.

Copper is dissolved by nitric acid and aqueous potassium cyanide.

Ruthenium can be dissolved in aqua regia, a highly concentrated mixture of hydrochloric acid and nitric acid, only when in the presence of oxygen, while rhodium must be in a fine pulverized form. Palladium and silver are soluble in nitric acid, while silver's solubility in aqua regia is limited by the formation of silver chloride precipitate.

Rhenium reacts with oxidizing acids, and hydrogen peroxide, and is said to be tarnished by moist air. Osmium and iridium are chemically inert in ambient conditions. Platinum and gold can be dissolved in aqua regia. Mercury reacts with oxidising acids.

In 2010, US researchers discovered that an organic "aqua regia" in the form of a mixture of thionyl chloride SOCl₂ and the organic solvent pyridine C₅H₅N achieved "high dissolution rates of noble metals under mild conditions, with the added benefit of being tunable to a specific metal" for example, gold but not palladium or platinum.

However, Gold can be dissolved in Selenic Acid (H₂SeO₄).

Anion (-ide) formation

The noble elements Gold and Platinum also have a comparatively high electronegativity for a metallic elements, thus alowing them to exist as single-metallic anions.

For example:

Cs + Au -> CsAu (Caesium Auride, a yellow crystalline salt with the Au
ion). Platinum also exhibits similar properties with BaPt, BaPt₂, Cs₂Pt (Barium and Caesium Platinides, which are reddish salts).

Electronic

The expression noble metal is sometimes confined to copper, silver, and gold since their full d-subshells can contribute to their noble character. There are also known to be significant contributions from how readily there is overlap of the d-electron states with the orbitals of other elements, particularly for gold. Relativistic contributions are also important, playing a role in the catalytic properties of gold.

The elements to the left of gold and silver have incompletely filled d-bands, which is believed to play a role in their catalytic properties. A common explanation is the d-band filling model of Hammer and Jens Nørskov, where the total d-bands are considered, not just the unoccupied states.

The low-energy plasmon properties are also of some importance, particularly those of silver and gold nanoparticles for surface-enhanced Raman spectroscopy, localized surface plasmons and other plasmonic properties.

Electrochemical

Standard reduction potentials in aqueous solution are also a useful way of predicting the non-aqueous chemistry of the metals involved. Thus, metals with high negative potentials, such as sodium, or potassium, will ignite in air, forming the respective oxides. These fires cannot be extinguished with water, which also react with the metals involved to give hydrogen, which is itself explosive. Noble metals, in contrast, are disinclined to react with oxygen and, for that reason (as well as their scarcity) have been valued for millennia, and used in jewellery and coins.

The adjacent table lists standard reduction potential in volts; electronegativity (revised Pauling); and electron affinity values (kJ/mol), for some metals and metalloids.

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 positive potentials; 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".

The black tarnish commonly seen on silver arises from its sensitivity to sulphur containing gases such as hydrogen sulfide:

2 Ag + H₂S + ⁠1/2⁠O₂ → Ag₂S + H₂O.

Rayner-Canham 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.

The relevance of the entry for water is addressed by Li et al. 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 phase with electrolyte exists, and (3) one of the two metals has...potential lower than the potential of the reaction (H
₂O + 4e + O
₂ = 4 OH) 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 H
₂O − 4 e − O
₂ = 4 OH)."

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. Copernicium's behaviour seems to partly resemble both its lighter congener mercury and the noble gas radon.

Oxides

As long ago as 1890, Hiorns observed as follows:

"Noble Metals. Gold, Platinum, Silver, and a few rare metals. The members of this class have little or no tendency to unite with oxygen in the free state, and when placed in water at a red heat do not alter its composition. The oxides are readily decomposed by heat in consequence of the feeble affinity between the metal and oxygen."

Smith, writing in 1946, continued the theme:

"There is no sharp dividing line but perhaps the best definition 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. 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.

Catalytic properties

All the noble metals can act as catalysts. For example, platinum is used in catalytic converters, devices which convert toxic gases produced in car engines, such as the oxides of nitrogen, into non-polluting substances.

Gold has many industrial applications; it is used as a catalyst in hydrogenation and the water gas shift reaction.

See also

• Galvanic series
• Minor metals
• Hallmark
• Precious metal

Notes

1. Palladium oxide PdO can be reduced to palladium metal by exposing it to hydrogen in ambient conditions
2. Ag₄O₄ is a mixed oxidation state compound silver in the oxidation state of 1 and 3.
3. Incipient red heat corresponds to 525 °C

References

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. ^ 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.
5. Everett Collier, "The Boatowner's Guide to Corrosion", International Marine Publishing, 2001, p. 21
6. "the definition of noble metal". Dictionary.com. Retrieved April 6, 2018.
7. Constable EC 2019, "Evolution and understanding of the d-block elements in the periodic table", Dalton Transactions, vol. 48, no. 26, pp. 9408-9421 doi:10.1039/C9DT00765B
8. W. Xing, M. Lee, Geosys. Eng. 20, 216, 2017
9. ^ Parish RV 1977, The metallic elements, Longman, London, p. 53, 115
10. ^ A. Holleman, N. Wiberg, "Inorganic Chemistry", Academic Press, 2001
11. Urquhart J 2010, "Challenging aqua regia's throne", Chemistry World, 24 September
12. Karpov, Andrey; Nuss, Jürgen; Wedig, Ulrich; Jansen, Martin (2003). "Cs2Pt: A Platinide(-II) Exhibiting Complete Charge Separation". Angewandte Chemie International Edition. 42 (39): 4818–21. doi:10.1002/anie.200352314. PMID 14562358.
13. Karpov, Andrey; Konuma, Mitsuharu; Jansen, Martin (2006). "An experimental proof for negative oxidation states of platinum: ESCA-measurements on barium platinides". Chemical Communications. 44 (8): 838–840. doi:10.1039/b514631c. PMID 16479284.
14. Ruban, A; Hammer, B; Stoltze, P; Skriver, H.L; Nørskov, J.K (1997). "Surface electronic structure and reactivity of transition and noble metals1Communication presented at the First Francqui Colloquium, Brussels, 19–20 February 1996.1". Journal of Molecular Catalysis A: Chemical. 115 (3): 421–429. doi:10.1016/S1381-1169(96)00348-2.
15. Hammer, B.; Norskov, J. K. (1995). "Why gold is the noblest of all the metals". Nature. 376 (6537): 238–240. Bibcode:1995Natur.376..238H. doi:10.1038/376238a0. ISSN 0028-0836.
16. Bartlett, Neil (1998). "Relativistic effects and the chemistry of gold". Gold Bulletin. 31 (1): 22–25. doi:10.1007/BF03215471. ISSN 0017-1557.
17. Gorin, David J.; Toste, F. Dean (March 22, 2007). "Relativistic effects in homogeneous gold catalysis". Nature. 446 (7134): 395–403. Bibcode:2007Natur.446..395G. doi:10.1038/nature05592. ISSN 0028-0836. PMID 17377576.
18. Hammer, B.; Nørskov, J.K. (1995). "Electronic factors determining the reactivity of metal surfaces". Surface Science. 343 (3): 211–220. Bibcode:1995SurSc.343..211H. doi:10.1016/0039-6028(96)80007-0.
19. Greeley, Jeff; Nørskov, Jens K.; Mavrikakis, Manos (2002). "Electronic Structure and Catalysis on Metal Surfaces". Annual Review of Physical Chemistry. 53 (1): 319–348. Bibcode:2002ARPC...53..319G. doi:10.1146/annurev.physchem.53.100301.131630. ISSN 0066-426X. PMID 11972011.
20. Garcia, M A (2011). "Surface plasmons in metallic nanoparticles: fundamentals and applications". Journal of Physics D: Applied Physics. 44 (28): 283001. Bibcode:2011JPhD...44B3001G. doi:10.1088/0022-3727/44/28/283001.
21. Zhang, Junxi; Zhang, Lide; Xu, Wei (March 21, 2012). "Surface plasmon polaritons: physics and applications". Journal of Physics D: Applied Physics. 45 (11): 113001. Bibcode:2012JPhD...45k3001Z. doi:10.1088/0022-3727/45/11/113001. ISSN 0022-3727.
22. G. Wulfsberg 2000, "Inorganic Chemistry", University Science Books, Sausalito, CA, pp. 270, 937.
23. 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
24. Ahmad, Z (2006). Principles of corrosion engineering and corrosion control. Amsterdam: Elsevier. p. 40. ISBN 9780080480336.
25. Powers, JM; Wataha, JE (2013). Dental materials: Properties and manipulation (10th ed.). St Louis: Elsevier Health Sciences. p. 134. ISBN 9780323291507.
26. Li, Y; Lu, D; Wong, CP (2010). Electrical conductive adhesives with nanotechnologies. New York: Springer. p. 179. ISBN 978-0-387-88782-1.
27. 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.
28. Mewes, J.-M.; Smits, O. R.; Kresse, G.; Schwerdtfeger, P. (2019). "Copernicium is a Relativistic Noble Liquid". Angewandte Chemie International Edition. 58 (50): 17964–17968. doi:10.1002/anie.201906966. PMC 6916354. PMID 31596013.
29. Hiorns AH 1890, Mixed metals or metallic alloys, p. 7
30. Hiorns RH 1890, Mixed metals or metallic alloys, MacMillian, New York, p. 5
31. Smith, JC (1946). The chemistry and metallurgy of dental materials. Oxford: Blackwell. p. 40.

Further reading

• Balshaw L 2020, "Noble metals dissolved without aqua regia", Chemistry World, 1 September
• Beamish FE 2012, The analytical chemistry of the noble metals, Elsevier Science, Burlington
• Brasser R, Mojzsis SJ 2017, "A colossal impact enriched Mars' mantle with noble metals", Geophys. Res. Lett., vol. 44, pp. 5978–5985, doi:10.1002/2017GL074002
• Brooks RR (ed.) 1992, Noble metals and biological systems: Their role in medicine, mineral exploration, and the environment, CRC Press, Boca Raton
• Brubaker PE, Moran JP, Bridbord K, Hueter FG 1975, "Noble metals: a toxicological appraisal of potential new environmental contaminants", Environmental Health Perspectives, vol. 10, pp. 39–56, doi:10.1289/ehp.751039
• Du R et al. 2019, "Emerging noble metal aerogels: State of the art and a look forward", Matter, vol. 1, pp. 39–56
• Hämäläinen J, Ritala M, Leskelä M 2013, "Atomic layer deposition of noble metals and their oxides", Chemistry of Materials, vol. 26, no. 1, pp. 786–801, doi:10.1021/cm402221
• Kepp K 2020, "Chemical causes of metal nobleness", ChemPhysChem, vol. 21 no. 5. pp. 360−369,doi:10.1002/cphc.202000013
• Lal H, Bhagat SN 1985, "Gradation of the metallic character of noble metals on the basis of thermoelectric properties", Indian Journal of Pure and Applied Physics, vol. 23, no. 11, pp. 551–554
• Lyon SB 2010, "3.21 - Corrosion of noble metals", in B Cottis et al. (eds.), Shreir's Corrosion, Elsevier, pp. 2205–2223, doi:10.1016/B978-044452787-5.00109-8
• Medici S, Peana MF, Zoroddu MA 2018, "Noble metals in pharmaceuticals: Applications and limitations", in M Rai M, Ingle, S Medici (eds.), Biomedical applications of metals, Springer, doi:10.1007/978-3-319-74814-6_1
• Pan S et al. 2019, "Noble-noble strong union: Gold at its best to make a bond with a noble gas atom", ChemistryOpen, vol. 8, p. 173, doi:10.1002/open.201800257
• Russel A 1931, "Simple deposition of reactive metals on noble metals", Nature, vol. 127, pp. 273–274, doi:10.1038/127273b0
• St. John J et al. 1984, Noble metals, Time-Life Books, Alexandria, VA
• Wang H 2017, "Chapter 9 - Noble Metals", in LY Jiang, N Li (eds.), Membrane-based separations in metallurgy, Elsevier, pp. 249–272, doi:10.1016/B978-0-12-803410-1.00009-8

External links

• Noble metal – chemistry Encyclopædia Britannica, online edition

• Chemical nomenclature
• Metallurgy
• Noble metals