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A noble metal is ordinarily regarded as a metallic
chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...
that is generally resistant to
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
and is usually found in nature in its raw form.
Gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
,
platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...
, and the other platinum group metals (
ruthenium Ruthenium is a chemical element with the symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most other chemic ...
, rhodium, palladium, osmium, iridium) are most often so classified.
Silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
,
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...
and mercury are sometimes included as noble metals, however less often as each of these usually occurs in nature combined with
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
. In more specialized fields of study and applications the number of elements counted as noble metals can be smaller or larger. In
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which rel ...
, there are only three noble metals:
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...
, silver and gold. In
dentistry Dentistry, also known as dental medicine and oral medicine, is the branch of medicine focused on the teeth, gums, and mouth. It consists of the study, diagnosis, prevention, management, and treatment of diseases, disorders, and conditions ...
, silver is not always counted as a noble metal since it is subject to corrosion when present in the mouth. In chemistry, the term noble metal 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 Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, bu ...
,
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient ti ...
,
bismuth Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...
and polonium, as well as gold, the six platinum group metals, and silver. __TOC__


Meaning and history

While noble metal lists can differ, they tend to cluster around the six platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, platinum) plus gold. In addition to this term's function as a compound
noun A noun () is a word that generally functions as the name of a specific object or set of objects, such as living creatures, places, actions, qualities, states of existence, or ideas.Example nouns for: * Organism, Living creatures (including people ...
, 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 Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on la ...
is more noble than silver and the relative nobility of many materials is highly dependent upon context, as for
aluminium Aluminium (aluminum in AmE, American and CanE, Canadian English) is a chemical element with the Symbol (chemistry), symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately o ...
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. File:Chalcopyrite-199453.jpg,
Chalcopyrite, which is copper iron sulfide (CuFeS2), is the most abundant copper ore mineral
File:Ruthenium a half bar.jpg, One half of a ruthenium bar.
Size ~ 40 × 15 × 10 mm
Weight ~44 g File:Rhodium powder pressed melted.jpg,
Rhodium: 1 g powder, 1g pressed cylinder, 1 g pellet.
File:Palladium (46 Pd).jpg,
Palladium
File:Acanthite - Imiter mine, Jbel Saghro, Tinghir, Drâa-Tafilalet, Morocco.jpg,
Acanthite Acanthite is a form of silver sulfide with the chemical formula Ag2S. It crystallizes in the monoclinic system and is the stable form of silver sulfide below . Argentite is the stable form above that temperature. As argentite cools below that te ...
, or silver sulfide (Ag2S), is the most important silver ore
File:Osmium crystals.jpg,
Osmium crystals, 2.2 g
File:Iridium-2.jpg,
Pieces of pure iridium, 1 g, size: 1–3 mm each
File:Platinum crystals.jpg,
Crystals of pure platinum
File:Gold nugget (Australia) 4 (16848647509).jpg,
Gold nugget from Australia, nearly 9,000 g or 64 oz
File:cinnabar09.jpg,
Cinnabar Cinnabar (), or cinnabarite (), from the grc, κιννάβαρι (), is the bright scarlet to brick-red form of mercury(II) sulfide (HgS). It is the most common source ore for refining elemental mercury and is the historic source for the bri ...
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 metal A native metal is any metal that is found pure in its metallic form in nature. Metals that can be found as native deposits singly or in alloys include aluminium, antimony, arsenic, bismuth, cadmium, chromium, cobalt, indium, iron, manganese, ...
s that occur naturally in relatively large amounts.


Corrosion resistance

Copper is dissolved by
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
and aqueous
potassium cyanide Potassium cyanide is a compound with the formula KCN. This colorless crystalline salt, similar in appearance to sugar, is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications i ...
. Ruthenium can be dissolved in aqua regia, a highly concentrated mixture of
hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dig ...
and
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
, only when in the presence of oxygen, while rhodium must be in a fine pulverized form. Palladium and silver are soluble in
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
, with the solubility of silver being limited by the formation of silver chloride precipitate.W. Xing, M. Lee, Geosys. Eng. 20, 216, 2017 Rhenium reacts with oxidizing acids, and
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
, and is said to be tarnished by moist air. Osmium and iridium are chemically inert in ambient conditions.Parish RV 1977, ''The metallic elements,'' Longman, London, p. 53, 115 Platinum and gold can be dissolved in aqua regia.A. Holleman, N. Wiberg, "Inorganic Chemistry", Academic Press, 2001 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 Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakl ...
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.


Electronic

In physics, the expression "noble metal" is sometimes confined to copper, silver, and gold, since their full d-subshells contribute to what noble character they have. In contrast, the other noble metals, especially the platinum group metals, have notable catalytic applications, arising from their partially filled d-subshells. This is the case with palladium which has a full d-subshell in the atomic state but in condensed form has a partially filled sp band at the expense of d-band occupancy. The difference in reactivity can 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 Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
very quickly.


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". On account of their high electron affinity values, the incorporation of a noble metal in the electrochemical photolysis process, such as platinum and gold, among others, can increase photoactivity. Arsenic and antimony are usually considered to be metalloids rather than noble metals. However, physically speaking their most stable allotropes are metallic. Semiconductors, such as selenium and tellurium, have been excluded. The black tarnish commonly seen on silver arises from its sensitivity to
hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The und ...
: :2 Ag + H2S + O2 → 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 Dentistry, also known as dental medicine and oral medicine, is the branch of medicine focused on the teeth, gums, and mouth. It consists of the study, diagnosis, prevention, management, and treatment of diseases, disorders, and conditions ...
, 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 ( + 4e + = 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 − 4 e• − = 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 Nihonium is a synthetic chemical element with the symbol Nh and atomic number 113. It is extremely radioactive; its most stable known isotope, nihonium-286, has a half-life of about 10 seconds. In the periodic table, nihonium is a transact ...
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 etween 'noble metals' and 'base metals'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

Many of the noble metals can act as catalysts. For example, platinum is used in
catalytic converter A catalytic converter is an exhaust emission control device that converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction. Catalytic converters are usuall ...
s, 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 Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate org ...
and the
water gas shift Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a s ...
reaction.


See also

* Minor metals *
Precious metal Precious metals are rare, naturally occurring metallic chemical elements of high economic value. Chemically, the precious metals tend to be less reactive than most elements (see noble metal). They are usually ductile and have a high lu ...


Notes


References


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, * 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, * 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, * Kepp K 2020, "Chemical causes of metal nobleness", ''ChemPhysChem,'' vol. 21 no. 5. pp. 360−369, * 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, * 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, * 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, * Russel A 1931, "Simple deposition of reactive metals on noble metals", ''Nature,'' vol. 127, pp. 273–274, * 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,


External links


Noble metal – chemistry
Encyclopædia Britannica, online edition {{Navbox periodic table Chemical nomenclature Metallurgy