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  often (Ag)  2  sometimes (Cu, Hg)  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.

---

* 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.

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

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.

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.

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.

Electrochemical properties of some metals and metalloids

Element	Z	G	P	Reaction	SRP(V)	EN	EA
Gold ✣	79	11	6	Au3+ + 3 e− → Au	1.5	2.54	223
Platinum ✣	78	10	6	Pt2+ + 2 e− → Pt	1.2	2.28	205
Iridium ✣	77	9	6	Ir3+ + 3 e− → Ir	1.16	2.2	151
Palladium ✣	46	10	5	Pd2+ + 2 e− → Pd	0.915	2.2	54
Osmium ✣	76	8	6	OsO 2 + 4 H+ + 4 e− → Os + 2 H 2O	0.85	2.2	104
Mercury	80	12	6	Hg2+ + 2 e− → Hg	0.85	2.0	−50
Rhodium ✣	45	9	5	Rh3+ + 3 e− → Rh	0.8	2.28	110
Silver ✣	47	11	5	Ag+ + e− → Ag	0.7993	1.93	126
Ruthenium ✣	44	8	5	Ru3+ + 3 e− → Ru	0.6	2.2	101
Polonium ☢	84	16	6	Po2+ + 2 e− → Po	0.6	2.0	136
Water				2 H 2O + 4 e− +O 2 → 4 OH−	0.4
Copper	29	11	4	Cu2+ + 2 e− → Cu	0.339	2.0	119
Bismuth	83	15	6	Bi3+ + 3 e− → Bi	0.308	2.02	91
Technetium ☢	43	7	6	TcO 2 + 4 H+ + 4 e− → Tc + 2 H 2O	0.28	1.9	53
Rhenium	75	7	6	ReO 2 + 4 H+ + 4 e− → Re + 2 H 2O	0.251	1.9	6
ArsenicMD	33	15	4	As 4O 6 + 12 H+ + 12 e− → 4 As + 6 H 2O	0.24	2.18	78
AntimonyMD	51	15	5	Sb 2O 3 + 6 H+ + 6 e− → 2 Sb + 3 H 2O	0.147	2.05	101
Z atomic number; G group; P period; SRP standard reduction potential; EN electronegativity; EA electron affinity
✣ traditionally recognized as a noble metal; MD metalloid; ☢ radioactive

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

Oxide melting points, °C

Element	I	II	III	IV	VI	VII	VIII
Copper	1232	1326
Ruthenium				d1300			25
Rhodium			d1100	d1050
Palladium		d750
Silver	d200	d100
Rhenium				d1000	d400	327
Osmium				d500			40
Iridium				d1100
Platinum				450
Gold			d150
Mercury		d500
Strontium‡		2430
Molybdenum‡					801
AntimonyMD			655
Lanthanum‡			2320
Bismuth‡			817
d = decomposes; ‡ = not a noble metal; MD = metalloid

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 [between '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

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

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