Rhodium is a chemical element with 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, produced after it reacted with the powerful acid mixture aqua regia.
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.
|Appearance||silvery white metallic|
|Standard atomic weight Ar, std(Rh)||102.90549(2)|
|Rhodium in the periodic table|
|Atomic number (Z)||45|
|Element category||transition metal|
|Electron configuration||[Kr] 4d8 5s1|
Electrons per shell
|2, 8, 18, 16, 1|
|Phase at STP||solid|
|Melting point||2237 K (1964 °C, 3567 °F)|
|Boiling point||3968 K (3695 °C, 6683 °F)|
|Density (near r.t.)||12.41 g/cm3|
|when liquid (at m.p.)||10.7 g/cm3|
|Heat of fusion||26.59 kJ/mol|
|Heat of vaporization||493 kJ/mol|
|Molar heat capacity||24.98 J/(mol·K)|
|Oxidation states||−3, −1, +1, +2, +3, +4, +5, +6 (an amphoteric oxide)|
|Electronegativity||Pauling scale: 2.28|
|Atomic radius||empirical: 134 pm|
|Covalent radius||142±7 pm|
Spectral lines of rhodium
|Crystal structure|| face-centered cubic (fcc)|
|Speed of sound thin rod||4700 m/s (at 20 °C)|
|Thermal expansion||8.2 µm/(m·K) (at 25 °C)|
|Thermal conductivity||150 W/(m·K)|
|Electrical resistivity||43.3 nΩ·m (at 0 °C)|
|Magnetic susceptibility||+111.0·10−6 cm3/mol (298 K)|
|Young's modulus||380 GPa|
|Shear modulus||150 GPa|
|Bulk modulus||275 GPa|
|Vickers hardness||1100–8000 MPa|
|Brinell hardness||980–1350 MPa|
|Discovery and first isolation||William Hyde Wollaston (1804)|
|Main isotopes of rhodium|
Rhodium (Greek rhodon (ῥόδον) meaning "rose") was discovered in 1803 by William Hyde Wollaston, soon after his discovery of palladium. He used crude platinum ore presumably obtained from South America. His procedure involved dissolving the ore in aqua regia and neutralizing the acid with sodium hydroxide (NaOH). He then precipitated the platinum as ammonium chloroplatinate by adding ammonium chloride (NH
4Cl). Most other metals like copper, lead, palladium and rhodium were precipitated with zinc. Diluted nitric acid dissolved all but palladium and rhodium. Of these, palladium dissolved in aqua regia but rhodium did not, and the rhodium was precipitated by the addition of sodium chloride as Na
2O. After being washed with ethanol, the rose-red precipitate was reacted with zinc, which displaced the rhodium in the ionic compound and thereby released the rhodium as free metal.
After the discovery, the rare element had only minor applications; for example, by the turn of the century, rhodium-containing thermocouples were used to measure temperatures up to 1800 °C. The first major application was electroplating for decorative uses and as corrosion-resistant coating. The introduction of the three-way catalytic converter by Volvo in 1976 increased the demand for rhodium. The previous catalytic converters used platinum or palladium, while the three-way catalytic converter used rhodium to reduce the amount of NOx in the exhaust.
|Z||Element||No. of electrons/shell|
|27||cobalt||2, 8, 15, 2|
|45||rhodium||2, 8, 18, 16, 1|
|77||iridium||2, 8, 18, 32, 15, 2|
|109||meitnerium||2, 8, 18, 32, 32, 15, 2 (predicted)|
Rhodium is a hard, silvery, durable metal that has a high reflectance. Rhodium metal does not normally form an oxide, even when heated. Oxygen is absorbed from the atmosphere only at the melting point of rhodium, but is released on solidification. Rhodium has both a higher melting point and lower density than platinum. It is not attacked by most acids: it is completely insoluble in nitric acid and dissolves slightly in aqua regia.
Rhodium belongs to group 9 of the periodic table, but the configuration of electrons in the outermost shells is atypical for the group. This anomaly is also observed in the neighboring elements, niobium (41), ruthenium (44), and palladium (46).
Unlike ruthenium and osmium, rhodium forms no volatile oxygen compounds. The known stable oxides include Rh
6 and Sr
6. Halogen compounds are known in nearly the full range of possible oxidation states. Rhodium(III) chloride, rhodium(IV) fluoride, rhodium(V) fluoride and rhodium(VI) fluoride are examples. The lower oxidation states are stable only in the presence of ligands.
Naturally occurring rhodium is composed of only one isotope, 103Rh. The most stable radioisotopes are 101Rh with a half-life of 3.3 years, 102Rh with a half-life of 207 days, 102mRh with a half-life of 2.9 years, and 99Rh with a half-life of 16.1 days. Twenty other radioisotopes have been characterized with atomic weights ranging from 92.926 u (93Rh) to 116.925 u (117Rh). Most of these have half-lives shorter than an hour, except 100Rh (20.8 hours) and 105Rh (35.36 hours). Rhodium has numerous meta states, the most stable being 102mRh (0.141 MeV) with a half-life of about 2.9 years and 101mRh (0.157 MeV) with a half-life of 4.34 days (see isotopes of rhodium).
In isotopes weighing less than 103 (the stable isotope), the primary decay mode is electron capture and the primary decay product is ruthenium. In isotopes greater than 103, the primary decay mode is beta emission and the primary product is palladium.
The industrial extraction of rhodium is complex because the ores are mixed with other metals such as palladium, silver, platinum, and gold and there are very few rhodium-bearing minerals. It is found in platinum ores and extracted as a white inert metal that is difficult to fuse. Principal sources are located in South Africa; in river sands of the Ural Mountains; and in North America, including the copper-nickel sulfide mining area of the Sudbury, Ontario, region. Although the quantity at Sudbury is very small, the large amount of processed nickel ore makes rhodium recovery cost-effective.
The main exporter of rhodium is South Africa (approximately 80% in 2010) followed by Russia. The annual world production is 30 tonnes. The price of rhodium is highly variable. In 2007, rhodium cost approximately eight times more than gold, 450 times more than silver, and 27,250 times more than copper by weight. In 2008, the price briefly rose above $10,000 per ounce ($350,000 per kilogram). The economic slowdown of the 3rd quarter of 2008 pushed rhodium prices sharply back below $1,000 per ounce ($35,000 per kilogram); the price rebounded to $2,750 by early 2010 ($97,000 per kilogram) (more than twice the gold price), but in late 2013, the prices were less than $1000.
Political and financial problems led to very low oil prices and oversupply, causing most metals to drop in price. The economies of China, India and other emerging countries slowed in 2014 and 2015. In 2014 alone, 23,722,890 motor vehicles were produced in China, excluding motorbikes. This resulted in a rhodium price of 740.00 US-$ per Troy ounce (31.1 grams) in late November 2015.
Rhodium is a fission product of uranium-235: each kilogram of fission product contains a significant amount of the lighter platinum group metals. Used nuclear fuel is therefore a potential source of rhodium, but the extraction is complex and expensive, and the presence of rhodium radioisotopes requires a period of cooling storage for multiple half-lives of the longest-lived isotope (about 10 years). These factors make the source unattractive and no large-scale extraction has been attempted.
The primary use of this element is in automobiles as a catalytic converter, changing harmful unburned hydrocarbons, carbon monoxide, and nitrogen oxide exhaust emissions into less noxious gases. Of 30,000 kg of rhodium consumed worldwide in 2012, 81% (24,300 kg) went into this application, and 8,060 kg was recovered from old converters. About 964 kg of rhodium was used in the glass industry, mostly for production of fiberglass and flat-panel glass, and 2,520 kg was used in the chemical industry.
Rhodium catalysts are used in a number of industrial processes, notably in catalytic carbonylation of methanol to produce acetic acid by the Monsanto process. It is also used to catalyze addition of hydrosilanes to molecular double bonds, a process important in manufacture of certain silicone rubbers. Rhodium catalysts are also used to reduce benzene to cyclohexane.
Rhodium finds use in jewelry and for decorations. It is electroplated on white gold and platinum to give it a reflective white surface at time of sale, after which the thin layer wears away with use. This is known as rhodium flashing in the jewelry business. It may also be used in coating sterling silver to protect against tarnish (silver sulfide, Ag2S, produced from atmospheric hydrogen sulfide, H2S). Solid (pure) rhodium jewelry is very rare, more because of the difficulty of fabrication (high melting point and poor malleability) than because of the high price. The high cost ensures that rhodium is applied only as an electroplate.
Rhodium has also been used for honors or to signify elite status, when more commonly used metals such as silver, gold or platinum were deemed insufficient. In 1979 the Guinness Book of World Records gave Paul McCartney a rhodium-plated disc for being history's all-time best-selling songwriter and recording artist.
Rhodium is used as an alloying agent for hardening and improving the corrosion resistance of platinum and palladium. These alloys are used in furnace windings, bushings for glass fiber production, thermocouple elements, electrodes for aircraft spark plugs, and laboratory crucibles. Other uses include:
Being a noble metal, pure rhodium is inert. However, chemical complexes of rhodium can be reactive. Median lethal dose (LD50) for rats is 198 mg of rhodium chloride (RhCl
3) per kilogram of body weight. Like the other noble metals, all of which are too inert to occur as chemical compounds in nature, rhodium has not been found to serve any biological function. In elemental form, the metal is harmless.
People can be exposed to rhodium in the workplace by inhalation. The Occupational Safety and Health Administration (OSHA) has specified the legal limit (Permissible exposure limit) for rhodium exposure in the workplace at 0.1 mg/m3 over an 8-hour workday, and the National Institute for Occupational Safety and Health (NIOSH) has set the recommended exposure limit (REL), at the same level. At levels of 100 mg/m3, rhodium is immediately dangerous to life or health. For soluble compounds, the PEL and REL are both 0.001 mg/m3.
Group 9, numbered by IUPAC nomenclature, is a group of chemical element in the periodic table. Members are cobalt (Co), rhodium (Rh), iridium (Ir) and perhaps also the chemically uncharacterized meitnerium (Mt). These are all transition metals in the d-block. All known isotopes of meitnerium are radioactive with short half-lives, and it is not known to occur in nature; only minute quantities have been synthesized in laboratories.
Like other groups, the members of this family show patterns in electron configuration, especially in the outermost shells, resulting in trends in chemical behavior; however, rhodium deviates from the pattern.HTC Touch Pro2
The HTC Touch Pro2 (also known as the AT&T Tilt 2, HTC Tytn III; codename: HTC Rhodium, HTC Barium, HTC Tungsten, HTC Fortress) is a slate smartphone, part of the Touch series of Internet-enabled, Windows Mobile, Pocket PC smartphones designed and marketed by HTC Corporation of Taiwan. It is an enhanced version of the HTC Touch Pro with a left-side slide-out QWERTY keyboard, with tilting screen. The Touch Pro2 smartphone's functions include those of a camera phone and a portable media player in addition to text messaging and multimedia messaging. It also offers Internet services including e-mail, instant messaging, web browsing, and local Wi-Fi connectivity. Visual voicemail is not a standard feature for the Touch Pro2, unlike its predecessor the Touch Pro. The Verizon Wireless version does include a visual voicemail application, however. All versions feature TouchFLO 3D — a new enhanced version of the TouchFLO interface, unique only to the latest Touch series. The latest update renamed TouchFLO 3D to SenseUI, to match HTC's Android offering. The Touch Pro2 — along with its sister model, the Touch Diamond2 — were unveiled on February 16, 2009 in Barcelona, Spain at the Mobile World Congress 2009. Specific enhancements over the original Touch Pro include:
Larger 3.6-inch WVGA display (0.2" smaller than the Touch HD)
Improved battery life
Conference calls with up to five other people using HTC Straight Talk, a new speaker phone system that includes two speakers and two microphones (for increased volume and noise cancellation, respectively)
Tiltable screenThe Touch Pro2 was released in May 2009.On August 6, 2009 Telus Mobility in Canada became the first North American carrier to launch the HTC Touch Pro2. The T-Mobile USA–branded Touch Pro2 was released on August 12, 2009. Sprint's version became available on September 8, 2009 and Verizon's version of the Touch Pro2 became available on September 11, 2009. The AT&T version branded Tilt 2 was released October 18, 2009 with Windows Mobile 6.5
As of November 2009, unofficial firmware for GSM and CDMA versions have been released by third parties. The XDAndroid project makes it possible to run Android on HTC Windows Mobile phones, including the Touch Pro2.
With a third party SIM card adaptor, the Pro2 is dual-SIM capable.
HTC manufactures Windows Mobile and Android-based Communicators which have a proprietary connector called HTC ExtUSB (Ext[ended] USB) which is present on the Touch Pro2. ExtUSB combines mini-USB (with which it is backwards-compatible) with audio/video input and output in an 11-pin connector.Hydrogenation
Hydrogenation – meaning, to treat with hydrogen – 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 organic compounds. Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to a molecule, often an alkene. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures. Hydrogenation reduces double and triple bonds in hydrocarbons.Isotopes of rhodium
Naturally occurring rhodium (45Rh) is composed of only one stable isotope, 103Rh. The most stable radioisotopes are 101Rh with a half-life of 3.3 years, 102Rh with a half-life of 207 days, and 99Rh with a half-life of 16.1 days. Thirty other radioisotopes have been characterized with atomic weights ranging from 88.949 u (89Rh) to 121.943 u (122Rh). Most of these have half-lifes that are less than an hour except 100Rh (half-life: 20.8 hours) and 105Rh (half-life: 35.36 hours). There are also numerous meta states with the most stable being 102mRh (0.141 MeV) with a half-life of about 3.7 years and 101mRh (0.157 MeV) with a half-life of 4.34 days.
The primary decay mode before the only stable isotope, 103Rh, is electron capture and the primary mode after is beta emission. The primary decay product before 103Rh is ruthenium and the primary product after is palladium.Meitnerium
Meitnerium is a synthetic chemical element with symbol Mt and atomic number 109. It is an extremely radioactive synthetic element (an element not found in nature, but can be created in a laboratory). The most stable known isotope, meitnerium-278, has a half-life of 7.6 seconds, although the unconfirmed meitnerium-282 may have a longer half-life of 67 seconds. The GSI Helmholtz Centre for Heavy Ion Research near Darmstadt, Germany, first created this element in 1982. It is named after Lise Meitner.
In the periodic table, meitnerium is a d-block transactinide element. It is a member of the 7th period and is placed in the group 9 elements, although no chemical experiments have yet been carried out to confirm that it behaves as the heavier homologue to iridium in group 9 as the seventh member of the 6d series of transition metals. Meitnerium is calculated to have similar properties to its lighter homologues, cobalt, rhodium, and iridium.Organorhodium chemistry
Organorhodium chemistry is the chemistry of organometallic compounds containing a rhodium-carbon chemical bond, and the study of rhodium and rhodium compounds as catalysts in organic reactions.Stable organorhodium compounds and transient organorhodium intermediates are used as catalyst such as in olefin hydroformylation, olefin hydrogenation, olefin isomerization and the Monsanto processPeriod 5 element
A period 5 element is one of the chemical elements in the fifth row (or period) of the periodic table of the elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The fifth period contains 18 elements, beginning with rubidium and ending with xenon. As a rule, period 5 elements fill their 5s shells first, then their 4d, and 5p shells, in that order; however, there are exceptions, such as rhodium.Platinum group
The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered together in the periodic table. These elements are all transition metals in the d-block (groups 8, 9, and 10, periods 5, 6 and 7).The six platinum-group metals are ruthenium, rhodium, palladium, osmium, iridium, and platinum. They have similar physical and chemical properties, and tend to occur together in the same mineral deposits. However they can be further subdivided into the iridium-group platinum-group elements (IPGEs: Os, Ir, Ru) and the palladium-group platinum-group elements (PPGEs: Rh, Pt, Pd) based on their behaviour in geological systems.The three elements above the platinum group in the periodic table (iron, nickel and cobalt) are all ferromagnetic, these being the only known transition metals with this property.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(II) acetate
Rhodium(II) acetate is the chemical compound with the formula Rh2(AcO)4, where AcO− is the acetate ion (CH3CO−2). This dark green powder is slightly soluble in polar solvents, including water. It is used as a catalyst for cyclopropanation of alkenes.Rhodium(III) chloride
Rhodium(III) chloride refers to inorganic compounds with the formula RhCl3(H2O)n, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh(III) centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt. The soluble salt is widely used to prepare compounds used in homogeneous catalysis, notably for the industrial production of acetic acid and hydroformylation.Rhodium(III) oxide
Rhodium(III) oxide (or Rhodium sesquioxide) is the inorganic compound with the formula Rh2O3. It is a gray solid that is insoluble in ordinary solvents.Rhodium(III) sulfide
Rhodium(III) sulfide is the inorganic compound with the formula Rh2S3. It is an insoluble black solid, prepared by the heating a mixture of elemental rhodium and sulfur. Crystals can be grown by chemical vapor transport using bromine as the transporting agent. The structure consists of octahedral and tetrahedral Rh and S centers, respectfully. No close Rh-Rh contacts are observed. Rh2Se3 and Ir2S3 adopt the same structure as Rh2S3.Rhodium(IV) oxide
Rhodium(IV) oxide (or rhodium dioxide) is the chemical compound with the formula RhO2.Rhodium hexafluoride
Rhodium hexafluoride, also rhodium(VI) fluoride, (RhF6) is the inorganic compound of rhodium and fluorine. A black volatile solid, it is a highly reactive material, and a rare example of a rhodium(VI) compound. It is one of seventeen known binary hexafluoride.Rhodocene
Rhodocene, formally known as bis(η5-cyclopentadienyl)rhodium(II), is a chemical compound with the formula [Rh(C5H5)2]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C5H5)2] radical is found above 150 °C or when trapped by cooling to liquid nitrogen temperatures (−196 °C). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.The history of organometallic chemistry includes the 19th-century discoveries of Zeise's salt and nickel tetracarbonyl. These compounds posed a challenge to chemists as the compounds did not fit with existing chemical bonding models. A further challenge arose with the discovery of ferrocene, the iron analogue of rhodocene and the first of the class of compounds now known as metallocenes. Ferrocene was found to be unusually chemically stable, as were analogous chemical structures including rhodocenium, the unipositive cation of rhodocene and its cobalt and iridium counterparts. The study of organometallic species including these ultimately led to the development of new bonding models that explained their formation and stability. Work on sandwich compounds, including the rhodocenium-rhodocene system, earned Geoffrey Wilkinson and Ernst Otto Fischer the 1973 Nobel Prize for Chemistry.Owing to their stability and relative ease of preparation, rhodocenium salts are the usual starting material for preparing rhodocene and substituted rhodocenes, all of which are unstable. The original synthesis used a cyclopentadienyl anion and tris(acetylacetonato)rhodium(III); numerous other approaches have since been reported, including gas-phase redox transmetalation and using half-sandwich precursors. Octaphenylrhodocene (a derivative with eight phenyl groups attached) was the first substituted rhodocene to be isolated at room temperature, though it decomposes rapidly in air. X-ray crystallography confirmed that octaphenylrhodocene has a sandwich structure with a staggered conformation. Unlike cobaltocene, which has become a useful one-electron reducing agent in research, no rhodocene derivative yet discovered is stable enough for such applications.
Biomedical researchers have examined the applications of rhodium compounds and their derivatives in medicine and reported one potential application for a rhodocene derivative as a radiopharmaceutical to treat small cancers. Rhodocene derivatives are used to synthesise linked metallocenes so that metal–metal interactions can be studied; potential applications of these derivatives include molecular electronics and research into the mechanisms of catalysis. The value of rhodocenes tends to be in the insights they provide into the bonding and dynamics of novel chemical systems, rather than their applications.Tetrarhodium dodecacarbonyl
Tetrarhodium dodecacarbonyl is the chemical compound with the formula Rh4(CO)12. This dark-red crystalline solid is the smallest stable binary rhodium carbonyl. It is used as a catalyst in organic synthesis.Wilkinson's catalyst
Wilkinson's catalyst, is the common name for chloridotris(triphenylphosphane)rhodium(I), a coordination complex of rhodium with the formula [RhCl(PPh3)3] (Ph = phenyl). It is a red-brown colored solid that is soluble in hydrocarbon solvents such as benzene, and more so in tetrahydrofuran or chlorinated solvents such as dichloromethane. The compound is widely used as a catalyst for hydrogenation of alkenes. It is named after chemist and Nobel Laureate, Sir Geoffrey Wilkinson, who first popularized its use.
Historically, Wilkinson's catalyst has been a paradigm in catalytic studies leading to several advances in the field such as the implementation of some of the first heteronuclear magnetic resonance studies for its structural elucidation in solution (31P), parahydrogen-induced polarization spectroscopy to determine the nature of transient reactive species, or one of the first detailed kinetic investigation by Halpern to elucidate the mechanism. Furthermore, the catalytic and organometallic studies on Wilkinson's catalyst also played a significant role on the subsequent development of cationic Rh- and Ru-based asymmetric hydrogenation transfer catalysts which set the foundations for modern asymmetric catalysis.William Hyde Wollaston
William Hyde Wollaston (; 6 August 1766 – 22 December 1828) was an English chemist and physicist who is famous for discovering the chemical elements palladium and rhodium. He also developed a way to process platinum ore into malleable ingots.