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).
More inclusive lists include one or more of mercury (Hg), rhenium (Re) 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.
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 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.
precious and semi-precious metalsOther
Non-precious unreactive metals
Radioactive unreactive metals
Radioactive, presumed unreactive metals
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.
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. 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.
|Element||Atomic number||Group||Period||Reaction||Potential||Electron configuration|
+ 2 e− → Cn
+ 3 e− → Rg
+ 2 e− → Ds
+ 3 e− → Au
+ e− → At
|Platinum||78||10||6||PtO + 2 H+
+ 2 e− → Pt + H
+ 2 e− → Pd
+ 2 e− → Fl
+ 3 e− → Mt
+ e− → Ag
2 + 2 e−→ 2 Hg
2 + 4 H+
+ 4 e− → Ir + 2 H
2 + 4 H+
+ 4 e− → Os + 2 H
+ 2 e− → Po
+ e− → Nh
+ 2 e− → Rh
+ 3 e− → Ru
2 + 4 H+
+ 4 e− → Te + 2 H
+ 4 e− → Hs
+ 2 e− → Cu
+ 3 e− → Bi
2 + 4 H+
+ 4 e− → Tc + 2 H
2 + 4 H+
+ 4 e− → Re + 2 H
6 + 12 H+
+ 12 e− → 4 As + 6 H
3 + 6 H+
+ 6 e− → 2 Sb + 3 H
+ 2 e− → Lv
+ 5 e− → Bh
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. On the other hand, rhenium-coated mirrors are said to be very durable, although rhenium and technetium are said to tarnish slowly in moist atmosphere.
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.
In pharmacology and medicine vectorization of drugs refers to (intracellular) targeting with plastic, noble metal or silicon nanoparticles or liposomes to which pharmacologically active substances are reversibly bound or attached by adsorption.CNRS researchers have devised a way to overcome the problem of multidrug resistance using polyalkylcyanoacrylate (PACA) nanoparticles as "vectors".Drug nanocarriers are expected to play a major role in delivering multiple drugs to tumor tissues by overcoming biological barriers.Erlichmanite
Erlichmanite is the naturally occurring mineral form of osmium sulfide (OsS2). It is grey with a metallic luster, hardness around 5, and specific gravity about 9. It is found in noble metal placer deposits. Named for Jozef Erlichman, electron microprobe analyst at the NASA Ames Research Center.Fushun Petrochemical Company
Fushun Petrochemical Company is a refining and petrochemical division of PetroChina. It is located in Fushun, Liaoning province, northeastern China. It is a manufacturer of different petrochemical products, as also catalysts for oil processing and noble metal refining. As of 2006, Fushun Petrochemicals was the world's largest producer of paraffin.In 2008, Fushun Petrochemical started to build a new refining and petrochemical complex in Fushun, Liaoning Province, China. This complex will include an ethylene, a polypropylene and a high density polyethylene plants. These plants are due to become operational by 2010.The plants are fed with oil from PetroChina's Daqing Field and import from Russia.Galvanic series
The galvanic series (or electropotential series) determines the nobility of metals and semi-metals. When two metals are submerged in an electrolyte, while also electrically connected by some external conductor, the less noble (base) will experience galvanic corrosion. The rate of corrosion is determined by the electrolyte, the difference in nobility, and the relative areas of the anode and cathode exposed to the electrolyte. The difference can be measured as a difference in voltage potential: the less noble metal is the one with a lower (that is, more negative) electrode potential than the nobler one, and will function as the anode (electron or anion attractor) within the electrolyte device functioning as described above (a galvanic cell). Galvanic reaction is the principle upon which batteries are based.
See the table of standard electrode potentials for more details.Hydrodesulfurization
Hydrodesulfurization (HDS) is a catalytic chemical process widely used to remove sulfur (S) from natural gas and from refined petroleum products, such as gasoline or petrol, jet fuel, kerosene, diesel fuel, and fuel oils. The purpose of removing the sulfur, and creating products such as ultra-low-sulfur diesel, is to reduce the sulfur dioxide (SO2) emissions that result from using those fuels in automotive vehicles, aircraft, railroad locomotives, ships, gas or oil burning power plants, residential and industrial furnaces, and other forms of fuel combustion.
Another important reason for removing sulfur from the naphtha streams within a petroleum refinery is that sulfur, even in extremely low concentrations, poisons the noble metal catalysts (platinum and rhenium) in the catalytic reforming units that are subsequently used to upgrade the octane rating of the naphtha streams.
The industrial hydrodesulfurization processes include facilities for the capture and removal of the resulting hydrogen sulfide (H2S) gas. In petroleum refineries, the hydrogen sulfide gas is then subsequently converted into byproduct elemental sulfur or sulfuric acid (H2SO4). In fact, the vast majority of the 64,000,000 metric tons of sulfur produced worldwide in 2005 was byproduct sulfur from refineries and other hydrocarbon processing plants.An HDS unit in the petroleum refining industry is also often referred to as a hydrotreater.Laser ablation synthesis in solution
Laser ablation synthesis in solution (LASiS) is a commonly used method for obtaining colloidal solution of nanoparticles in a variety of solvents.In the LASiS method, nanoparticles are produced during the condensation of a plasma plume formed by the laser ablation of a bulk metal plate dipped in a liquid solution. LASiS is usually considered a top-down physical approach. In the past years, laser ablation synthesis in solution (LASiS) emerged as a reliable alternative to traditional chemical reduction methods for obtaining noble metal nanoparticles (NMNp).
LASiS is a technique for the synthesis of stable NMNp in water or in organic solvents, which does not need stabilizing molecules or other chemicals. The so obtained NMNp are highly available for further functionalization or can be used wherever unprotected metal nanoparticles are desired.
Surface functionalization of NMNp can be monitored in real time by UV-visible spectroscopy of the plasmon resonance. However, LASiS has some limitations in the size control of NMNp, which can be overcome by laser treatments of NMNp.Localized surface plasmon
A localized surface plasmon (LSP) is the result of the confinement of a surface plasmon in a nanoparticle of size comparable to or smaller than the wavelength of light used to excite the plasmon. The LSP has two important effects: electric fields near the particle’s surface are greatly enhanced and the particle’s optical absorption has a maximum at the plasmon resonant frequency. The enhancement falls off quickly with distance from the surface and, for noble metal nanoparticles, the resonance occurs at visible wavelengths. For semiconductor nanoparticles, the maximum optical absorption is often in the near-infrared and mid-infrared region.Names for sets of chemical elements
There are currently 118 known chemical elements exhibiting a large number of different physical and chemical properties. Amongst this diversity, scientists have found it useful to use names for various sets of elements, that illustrate similar properties, or their trends of properties. Many of these sets are formally recognized by the standards body IUPAC.The following collective names are recommended by IUPAC:
Alkali metals – The metals of group 1: Li, Na, K, Rb, Cs, Fr.
Alkaline earth metals – The metals of group 2: Be, Mg, Ca, Sr, Ba, Ra.
Pnictogens – The elements of group 15: N, P, As, Sb, Bi. (Mc had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.)
Chalcogens – The elements of group 16: O, S, Se, Te, Po. (Lv had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.)
Halogens – The elements of group 17: F, Cl, Br, I, At. (Ts had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.)
Noble gases – The elements of group 18: He, Ne, Ar, Kr, Xe, Rn. (Og had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.)
Lanthanoids – Elements 57–71: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.
Actinoids – Elements 89–103: Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr.
Rare-earth metals – Sc, Y, plus the lanthanoids.
Transition elements – Elements in groups 3 to 11 or 3 to 12.Another common classification is by degree of metallic – metalloidal – nonmetallic behaviour and characteristics. There is no general agreement on the name to use for these sets: in this English Wikipedia, the name used is "category". Very often these categories are marked by a background color in the periodic table. Category names used here, without any claim to universality, are:
Alkali metals, alkaline earth metals, and noble gases: Same as the IUPAC system above.
Transition elements are instead referred to as transition metals.
Lanthanoids and actinoids are instead referred to as lanthanides and actinides respectively.
Rare-earth elements, pnictogens, chalcogens, and halogens are not used as category names, but the latter three are valid as group (column) names.
Additional element category names used:
Post-transition metals – The metals of groups 12–17: Zn, Cd, Hg, Al, Ga, In, Tl, Sn, Pb, Bi, Po. The period 7 elements Nh, Fl, Mc, Lv, and Ts are additionally predicted to be post-transition metals.
Metalloids – Elements with properties intermediate between metals and non-metals: B, Si, Ge, As, Sb, Te, At.
Reactive nonmetals – Nonmetals that are chemically active (as opposed to noble gases): H, C, N, P, O, S, Se, F, Cl, Br, I
Superactinides – Hypothetical series of elements 121 to 157, which includes a predicted "g-block" of the periodic table.Many other names for sets of elements are in common use, and yet others have been used throughout history. These sets usually do not aim to cover the whole periodic table (as for example period does). Some examples:
Precious metals – Variously-defined group of non-radioactive metals of high economical value.
Coinage metals – Various metals used to mint coins, primarily the group 11 elements Cu, Ag, and Au.
Platinum group – Ru, Rh, Pd, Os, Ir, Pt.
Noble metal – Variously-defined group of metals that are generally resistant to corrosion. Usually includes Ag, Au, and the platinum-group metals.
Heavy metals – Variously-defined group of metals, on the base of their density, atomic number, or toxicity.
Native metals – Metals that occur pure in nature, including the noble metals and others such as Sn and Pb.
Earth metal – Old historic term, usually referred to the metals of groups 3 and 13, although sometimes others such as beryllium and chromium are included as well.
Transuranium elements – Elements with atomic number greater than 92.
Transactinide elements – Elements after the actinides (atomic number greater than 103).
Transplutonium elements – Elements with atomic number greater than 94.
Minor actinides – Actinides found in significant quantities in nuclear fuel, other than U and Pu: Np, Am, Cm.
Heavy atom – term used in computational chemistry to refer to any element other than hydrogen and helium.Orichalcum
Orichalcum or aurichalcum is a metal mentioned in several ancient writings, including the story of Atlantis in the Critias of Plato. Within the dialogue, Critias (460 – 403 BC) claims that orichalcum had been considered second only to gold in value and had been found and mined in many parts of Atlantis in ancient times, but that by Critias' own time orichalcum was known only by name.
Orichalcum may have been a noble metal such as platinum, as it was supposed to be mined, or one type of bronze or brass or possibly some other metal alloy. In 2015, metal ingots were found in an ancient shipwreck in Gela (Sicily), which were made of an alloy primarily consisting of copper, zinc and small percentages of nickel, lead, and iron.In numismatics, orichalcum is the golden-colored bronze alloy used by the Roman Empire for their sestertius and dupondius coins.Passivation (chemistry)
Passivation, in physical chemistry and engineering, refers to a material becoming "passive," that is, less affected or corroded by the environment of future use. Passivation involves creation of an outer layer of shield material that is applied as a microcoating, created by chemical reaction with the base material, or allowed to build from spontaneous oxidation in the air. As a technique, passivation is the use of a light coat of a protective material, such as metal oxide, to create a shell against corrosion. Passivation can occur only in certain conditions, and is used in microelectronics to enhance silicon. The technique of passivation strengthens and preserves the appearance of metallics. In electrochemical treatment of water, passivation reduces the effectiveness of the treatment by increasing the circuit resistance, and active measures are typically used to overcome this effect, the most common being polarity reversal, which results in limited rejection of the fouling layer. Other proprietary systems to avoid electrode passivation, several discussed below, are the subject of ongoing research and development.
When exposed to air, many metals naturally form a hard, relatively inert surface, as in the tarnish of silver. In the case of other metals, such as iron, a somewhat rough porous coating is formed from loosely adherent corrosion products. In this case, a substantial amount of metal is removed, which is either deposited or dissolved in the environment. Corrosion coating reduces the rate of corrosion by varying degrees, depending on the kind of base metal and its environment, and is notably slower in room-temperature air for aluminium, chromium, zinc, titanium, and silicon (a metalloid); the shell of corrosion inhibits deeper corrosion, and operates as one form of passivation. The inert surface layer, termed the "native oxide layer", is usually an oxide or a nitride, with a thickness of a monolayer of 0.1-0.3 nm (1-3 Å) for a noble metal such as platinum, about 1.5 nm (15 Å) for silicon, and nearer to 5 nm (50 Å) for aluminium after several years.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 is derived from the Spanish term platino, meaning "little silver".Platinum is a member of the platinum group of elements and group 10 of the periodic table of elements. It has six naturally occurring isotopes. It is one of the rarer elements in Earth's crust, with an average abundance of approximately 5 μg/kg. It occurs in some nickel and copper ores along with some native deposits, mostly in South Africa, which accounts for 80% of the world production. Because of its scarcity in Earth's crust, only a few hundred tonnes are produced annually, and given its important uses, it is highly valuable and is a major precious metal commodity.Platinum is one of the least reactive metals. It has remarkable resistance to corrosion, even at high temperatures, and is therefore considered a noble metal. Consequently, platinum is often found chemically uncombined as native platinum. Because it occurs naturally in the alluvial sands of various rivers, it was first used by pre-Columbian South American natives to produce artifacts. It was referenced in European writings as early as 16th century, but it was not until Antonio de Ulloa published a report on a new metal of Colombian origin in 1748 that it began to be investigated by scientists.
Platinum is used in catalytic converters, laboratory equipment, electrical contacts and electrodes, platinum resistance thermometers, dentistry equipment, and jewelry. Being a heavy metal, it leads to health problems upon exposure to its salts; but due to its corrosion resistance, metallic platinum has not been linked to adverse health effects. Compounds containing platinum, such as cisplatin, oxaliplatin and carboplatin, are applied in chemotherapy against certain types of cancer.As of 2018, the value of platinum is $833.00 per ounce.Precious metal
A precious metal is a rare, naturally occurring metallic chemical element 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 lustre. Historically, precious metals were important as currency but are now regarded mainly as investment and industrial commodities. Gold, silver, platinum, and palladium each have an ISO 4217 currency code.
The best known precious metals are the coinage metals, which are gold and silver. Although both have industrial uses, they are better known for their uses in art, jewelry, and coinage. Other precious metals include the platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum, of which platinum is the most widely traded.
The demand for precious metals is driven not only by their practical use but also by their role as investments and a store of value. Historically, precious metals have commanded much higher prices than common industrial metals.Rhodium
Rhodium is a chemical element with the symbol Rh and atomic number 45. It is a rare, silvery-white, hard, corrosion-resistant, and chemically inert transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring isotope, 103Rh. Naturally occurring rhodium is usually found as the free metal, alloyed with similar metals, and rarely as a chemical compound in minerals such as bowieite and rhodplumsite. It is one of the rarest and most valuable precious metals.
Rhodium is found in platinum or nickel ores together with the other members of the platinum group metals. It was discovered in 1803 by William Hyde Wollaston in one such ore, and named for the rose color of one of its chlorine compounds.
The element's major use (approximately 80% of world rhodium production) is as one of the catalysts in the three-way catalytic converters in automobiles. Because rhodium metal is inert against corrosion and most aggressive chemicals, and because of its rarity, rhodium is usually alloyed with platinum or palladium and applied in high-temperature and corrosion-resistive coatings. White gold is often plated with a thin rhodium layer to improve its appearance while sterling silver is often rhodium-plated for tarnish resistance. Rhodium is sometimes used to cure silicones, a two part silicone where one part contains a silicon hydride and the other containing a vinyl terminated silicone are mixed. One of these liquids contains a rhodium complex.Rhodium detectors are used in nuclear reactors to measure the neutron flux level. Other uses of rhodium include asymmetric hydrogenation used to form drug precursors and the processes for the production of roundup and acetic acid.Selective leaching
Selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from the material. The less noble metal is removed from the alloy by a microscopic-scale galvanic corrosion mechanism. The most susceptible alloys are the ones containing metals with high distance between each other in the galvanic series, e.g. copper and zinc in brass. The elements most typically undergoing selective removal are zinc, aluminium, iron, cobalt, chromium, and others.Spent nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and depending on its point along the nuclear fuel cycle, it may have considerably different isotopic constituents.Tetraxenonogold(II)
Tetraxenonogold(II), gold tetraxenide(II) or AuXe2+4 is a cationic complex with a square planar configuration of atoms. It is found in the compound AuXe2+4(Sb2F−11)2, which exists in triclinic and tetragonal crystal modifications. The AuXe2+4 ion is stabilised by interactions with the fluoride atoms of the counterion. The Au-Xe bond length is 274 pm = 2.74 angstroms.Tetraxenonogold(II) is unusual in that it is a compound of the notoriously inert atoms xenon and gold. It is also unusual in that it uses xenon as a transition metal ligand, and in that it contains gold in the +2 oxidation state. It can be produced by reduction of AuF3 in the presence of fluoroantimonic acid and xenon, and crystallised at low temperature. The xenon bonds with the gold(II) ion to make this complex.
It was the first description of a compound between a noble gas and a noble metal. It was first described in the year 2000 by Konrad Seppelt and Stefan Seidel.Thalappil Pradeep
Thalappil Pradeep is an Institute professor and Professor of chemistry in the Department of Chemistry at the Indian Institute of Technology Madras. He is also an Institute Chair Professor.Thermocouple
A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming electrical junctions at differing temperatures. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature. Thermocouples are a widely used type of temperature sensor.Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is precision; system errors of less than one degree Celsius (°C) can be difficult to achieve.Thermocouples are widely used in science and industry. Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes. Thermocouples are also used in homes, offices and businesses as the temperature sensors in thermostats, and also as flame sensors in safety devices for gas-powered appliances.Tungsten trioxide
Tungsten(VI) oxide, also known as tungsten trioxide or tungstic anhydride, WO3, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce WO3. Further reaction with carbon or hydrogen gas reduces tungsten trioxide to the pure metal. Tungsten trioxide is a strong oxidative agent, it reacts rare-earth elements, iron, copper, aluminium, manganese, zinc, chromium, molybdenum, carbon, hydrogen and silver to make the pure tungsten metal, and gold and platinum to make the tungsten dioxide.
2 WO3 + 3 C → 2 W + 3 CO2 (high temperature)
WO3 + 3 H2 → W + 3 H2O (550 - 850 °C)
WO3 + 2Fe → W + Fe2O3
2WO3 + Pt → 2WO2 + PtO2Tungsten(VI) oxide occurs naturally in the form of hydrates, which include minerals: tungstite WO3·H2O, meymacite WO3·2H2O and hydrotungstite (of the same composition as meymacite, however sometimes written as H2WO4). These minerals are rare to very rare secondary tungsten minerals.
|Periodic table forms|
|Sets of elements|