Noble metal

In chemistry, the noble metals are metals that are resistant to corrosion and oxidation in moist air (unlike most base metals). The short list of chemically noble metals (those elements upon which almost all chemists agree) comprises ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au).[1]

Noble metals in the periodic table
H He
LiBe BCNOFNe
NaMg AlSiPSClAr
KCaScTiVCrMnFeCoNiCuZnGaGe As SeBrKr
RbSrYZrNbMoTcRuRhPdAgCdInSnSb TeIXe
CsBaLa*HfTaWReOsIrPtAuHgTlPbBiPoAtRn
FrRaAc**RfDbSgBhHsMtDsRgCnNhFlMcLvTsOg
 
*CePrNdPmSmEuGdTbDyHoErTmYbLu
**ThPaUNpPuAmCmBkCfEsFmMdNoLr
   Refractory metals
   Platinum group
   Other precious and semi-precious metals
   Non-precious unreactive metals
   Radioactive unreactive metals
   Radioactive, presumed unreactive metals

More inclusive lists include one or more of mercury (Hg),[2][3][4] rhenium (Re),[5] and copper (Cu) 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, rhenium and mercury, which are included by some definitions. These are arranged according to their position in the periodic table.

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[6] 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. For this reason, there are many quite different lists of "noble metals".

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.[7]

Properties

Platinum, gold and mercury can be dissolved in aqua regia, a highly concentrated mixture of hydrochloric acid and nitric acid, but iridium and silver cannot. 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]

Physics

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.[9] 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.[10]

Electrochemistry

Metallic elements, including metalloids (metals usually considered noble bolded, predictions for superheavy elements italicised):[11][12]

ElementAtomic numberGroupPeriodReactionPotentialElectron configuration
Copernicium112127Cn2+
 + 2 e− → Cn		2.1 V[Rn]5f146d107s2
Roentgenium111117Rg3+
 + 3 e− → Rg		1.9 V[Rn]5f146d97s2
Darmstadtium110107Ds2+
 + 2 e− → Ds		1.7 V[Rn]5f146d87s2
Gold79116Au3+
 + 3 e− → Au		1.5 V[Xe]4f145d106s1
Astatine85176At+
 + e− → At		1.0 V[Xe]4f145d106s26p5
Platinum78106PtO + 2 H+
 + 2 e− → Pt + H
2O		0.98 V[Xe]4f145d96s1
Palladium46105Pd2+
 + 2 e− → Pd		0.915 V[Kr]4d10
Flerovium114147Fl2+
 + 2 e− → Fl		0.9 V[Rn]5f146d107s27p2
Meitnerium10997Mt3+
 + 3 e− → Mt		0.8 V[Rn]5f146d77s2
Silver47115Ag+
 + e− → Ag		0.7993 V[Kr]4d105s1
Mercury80126Hg2+
2 + 2 e−→ 2 Hg		0.7925 V[Xe]4f145d106s2
Iridium7796IrO
2 + 4 H+
 + 4 e− → Ir + 2 H
2O		0.73 V[Xe]4f145d76s2
Osmium7686OsO
2 + 4 H+
 + 4 e− → Os + 2 H
2O		0.65 V[Xe]4f145d66s2
Polonium84166Po2+
 + 2 e− → Po		0.6 V[Xe]4f145d106s26p4
Nihonium113137Nh+
 + e− → Nh		0.6 V[Rn]5f146d107s27p1
Rhodium4595Rh2+
 + 2 e− → Rh		0.60 V[Kr]4d85s1
Ruthenium4485Ru3+
 + 3 e− → Ru		0.60 V[Kr]4d75s1
Tellurium52165TeO
2 + 4 H+
 + 4 e− → Te + 2 H
2O		0.57 V[Kr]4d105s25p4
Hassium10887Hs4+
 + 4 e− → Hs		0.4 V[Rn]5f146d67s2
Copper29114Cu2+
 + 2 e− → Cu		0.339 V[Ar]3d104s1
Bismuth83156Bi3+
 + 3 e− → Bi		0.308 V[Xe]4f145d106s26p3
Rhenium7576ReO
2 + 4 H+
 + 4 e− → Re + 2 H
2O		0.276 V[Xe]4f145d56s2
Technetium4375TcO
2 + 4 H+
 + 4 e− → Tc + 2 H
2O		0.272 V[Kr]4d55s2
Arsenic33154As
4O
6 + 12 H+
 + 12 e− → 4 As + 6 H
2O		0.24 V[Ar]3d104s24p3
Antimony51155Sb
2O
3 + 6 H+
 + 6 e− → 2 Sb + 3 H
2O		0.147 V[Kr]4d105s25p3
Livermorium116167Lv2+
 + 2 e− → Lv		0.1 V[Rn]5f146d107s27p4
Bohrium10777Bh5+
 + 5 e− → Bh		0.1 V[Rn]5f146d57s2

The columns group and period denote its position in the periodic table, hence electronic configuration. The simplified reactions, listed in the next column, can also be read in detail from the Pourbaix diagrams of the considered element in water. Finally the column potential indicates the electric potential of the element measured against a Standard hydrogen electrode. All missing elements in this table are either not metals or have a negative standard potential.

Arsenic, antimony and tellurium are considered to be metalloids and thus cannot be noble metals. Also, chemists and metallurgists consider copper and bismuth to not be noble metals because they easily oxidize due to the reaction O
2 + 2 H
2O + 4e− ⇄ 4 OH−
(aq) + 0.40 V which is possible in moist air.

The film of silver is due to its high sensitivity to hydrogen sulfide. Chemically, patina is caused by an attack of oxygen in wet air and by CO
2 afterward.[8] On the other hand, rhenium-coated mirrors are said to be very durable,[8] although rhenium and technetium are said to tarnish slowly in moist atmosphere.[13]

The superheavy elements from hassium to livermorium inclusive are expected to be "partially very noble metals"; chemical investigations of hassium and copernicium have established that they behave like their lighter homologs, the noble osmium and mercury, and preliminary investigations of nihonium and flerovium have suggested but not definitively established noble behavior.[14]

See also
  • Minor metals

References
  • Brooks, Robert R., ed. (1992). Noble Metals and Biological Systems: Their Role in Medicine, Mineral Exploration, and the Environment. Boca Raton, Fla.: CRC Press. ISBN 9780849361647. OCLC 24379749.
Notes
  1. A. Holleman, N. Wiberg, "Lehrbuch der Anorganischen Chemie", de Gruyter, 1985, 33. edition, p. 1486
  2. "Edelmetall". www.uni-protokolle.de. Retrieved April 6, 2018.
  3. "Dictionary of Mining, Mineral, and Related Terms", Compiled by the American Geological Institute, 2nd edition, 1997
  4. Scoullos, M.J., Vonkeman, G.H., Thornton, I., Makuch, Z., "Mercury – Cadmium – Lead: Handbook for Sustainable Heavy Metals Policy and Regulation",Series: Environment & Policy, Vol. 31, Springer-Verlag, 2002
  5. The New Encyclopædia Britannica, 15th edition, Vol. VII, 1976
  6. "the definition of noble metal". Dictionary.com. Retrieved April 6, 2018.
  7. Everett Collier, "The Boatowner’s Guide to Corrosion", International Marine Publishing, 2001, p. 21
  8. A. Holleman, N. Wiberg, "Inorganic Chemistry", Academic Press, 2001
  9. 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.
  10. 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
  11. 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
  12. 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.
  13. R. D. Peack, "The Chemistry of Technetium and Rhenium", Elsevier, 1966
  14. 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.
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