|Pronunciation|| /ˈroʊdiəm/ |
|Appearance||silvery white metallic|
|Standard atomic weight Ar°(Rh)|
|Rhodium in the periodic table|
|Atomic number (Z)||45|
|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)|
| Vapor pressure |
|Oxidation states||−3  , −1, 0, +1,  +2, +3, +4, +5, +6, +7  (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 ordering||paramagnetic |
|Molar 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)|
|Isotopes of rhodium|
Rhodium is a chemical element with the symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant 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 a free metal or as an alloy 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 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 (consuming about 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 to resist tarnishing. Rhodium is sometimes used to cure silicones: a two-part silicone in which one part containing 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 acetic acid.
Rhodium (Greek rhodon (ῥόδον) meaning "rose") was discovered in 1803 by William Hyde Wollaston,  soon after he discovered palladium.    He used crude platinum ore presumably obtained from South America.  His procedure dissolved the ore in aqua regia and neutralized 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. 
For decades, 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.   They have exceptionally good stability in the temperature range of 1300 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).
The common oxidation state of rhodium is +3, but oxidation states from 0 to +7 are also observed.  
Unlike ruthenium and osmium, rhodium forms no volatile oxygen compounds. The known stable oxides include Rh
3 , RhO
2 , RhO
6 and Sr
6.  Halogen compounds are known in nearly the full range of possible oxidation states. Rhodium(III) chloride, rhodium trifluoride, rhodium pentafluoride and rhodium hexafluoride are examples. The lower oxidation states are stable only in the presence of ligands. 
The best-known rhodium-halogen compound is the Wilkinson's catalyst chlorotris(triphenylphosphine)rhodium(I). This catalyst is used in the hydroformylation or hydrogenation of alkenes. 
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. 
Rhodium is one of the rarest elements in the Earth's crust, comprising an estimated 0.0002 parts per million (2 × 10−10).  Its rarity affects its price and its use in commercial applications. The concentration of rhodium in nickel meteorites is typically 1 part per billion.  Rhodium has been measured in some potatoes with concentrations between 0.8 and 30 ppt. 
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 in Russia; and in North America, including the copper-nickel sulfide mining area of the Sudbury, Ontario, region. Although the rhodium abundance 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 $1,000. Political and financial problems[ clarification needed ] led to very low oil prices and over supply, causing most metals to drop in price. The economies of China, India and other emerging countries slowed in 2014 and 2015. In 2014 alone, 23,722,890 motor vehicles were produced in China, excluding motorbikes.[ clarification needed ] This resulted in a rhodium price of 740.00 US-$ per Troy ounce (31.1 grams) in late November 2015. 
Owners of rhodium—a metal with a highly volatile market price—are periodically put in an extremely advantageous market position: extracting more rhodium-containing ore from the ground will necessarily also extract other much more abundant precious metals—notably platinum and palladium—which would oversupply the market with those other metals, lowering their prices. Since it is economically infeasible to simply extract these other metals just to obtain rhodium, the market is often left hopelessly squeezed for rhodium supply, causing prices to spike. Recovery from this supply-deficit position may be quite problematic in the future for many reasons, notably because it is not known how much rhodium (and other precious metals) actually was placed in catalytic converters during the many years when manufacturers' emissions-cheating software was in use. Much of the world supply of rhodium is obtained from recycled catalytic converters obtained from scrapped vehicles. As of early November 2020, the spot price of rhodium was US$14,700 per troy ounce.[ citation needed ] In early March 2021, rhodium reached a price of US$29,400 per troy ounce[ citation needed ] on Metals Daily[ unreliable source? ] (a precious metals commodity listing).
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 (101Rh with a half-life of 3.3 years, and 102mRh with a half-life of 2.9 years), or 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 is preferable to the other platinum metals in the reduction of nitrogen oxides to nitrogen and oxygen: 
In 2008, net demand (with the recycling accounted for) of rhodium for automotive converters made up 84% of the world usage,  with the number fluctuating around 80% in 2015−2021. 
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. 
The complex of a rhodium ion with BINAP is a widely used chiral catalyst for chiral synthesis, as in the synthesis of menthol. 
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:
In automobile manufacturing, rhodium is also used in the construction of headlight reflectors. 
|P273, P501 |
|NFPA 704 (fire diamond)|
Being a noble metal, pure rhodium is inert and harmless in elemental form.  However, chemical complexes of rhodium can be reactive. For rhodium chloride, the median lethal dose (LD50) for rats is 198 mg (RhCl
3) per kilogram of body weight.  Like the other noble metals, rhodium has not been found to serve any biological function.
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. 
Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal with a density of 22.56 g/cm3 (0.815 lb/cu in) as defined by experimental X-ray crystallography. It is one of the most corrosion-resistant metals, even at temperatures as high as 2,000 °C (3,630 °F). However, corrosion-resistance is not quantifiable in absolute terms; although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable, whereas gold dust is not flammable but can be attacked by substances that iridium resists, such as aqua regia.
Osmium is a chemical element with the symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mostly in platinum ores. Osmium is the densest naturally occurring element. When experimentally measured using X-ray crystallography, it has a density of 22.59 g/cm3. Manufacturers use its alloys with platinum, iridium, and other platinum-group metals to make fountain pen nib tipping, electrical contacts, and in other applications that require extreme durability and hardness.
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired by her when she slew Pallas. Palladium, platinum, rhodium, ruthenium, iridium and osmium form a group of elements referred to as the platinum group metals (PGMs). They have similar chemical properties, but palladium has the lowest melting point and is the least dense of them.
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 platina, a diminutive of plata "silver".
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 chemicals. Russian-born scientist of Baltic-German ancestry Karl Ernst Claus discovered the element in 1844 at Kazan State University and named ruthenium in honor of Russia. Ruthenium is usually found as a minor component of platinum ores; the annual production has risen from about 19 tonnes in 2009 to some 35.5 tonnes in 2017. Most ruthenium produced is used in wear-resistant electrical contacts and thick-film resistors. A minor application for ruthenium is in platinum alloys and as a chemistry catalyst. A new application of ruthenium is as the capping layer for extreme ultraviolet photomasks. Ruthenium is generally found in ores with the other platinum group metals in the Ural Mountains and in North and South America. Small but commercially important quantities are also found in pentlandite extracted from Sudbury, Ontario and in pyroxenite deposits in South Africa.
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one of the rarest elements in the Earth's crust. Rhenium has the third-highest melting point and second-highest boiling point of any element at 5869 K. Rhenium resembles manganese and technetium chemically and is mainly obtained as a by-product of the extraction and refinement of molybdenum and copper ores. Rhenium shows in its compounds a wide variety of oxidation states ranging from −1 to +7.
A period 5 element is one of the chemical elements in the fifth row of the periodic table of the chemical 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.
A noble metal is ordinarily regarded as a metallic chemical 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 are most often so classified. Silver, copper and mercury are sometimes included as noble metals, however less often as each of these usually occurs in nature combined with sulfur.
A period 6 element is one of the chemical elements in the sixth row (or period) of the periodic table of the chemical elements, including the lanthanides. 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 sixth period contains 32 elements, tied for the most with period 7, beginning with caesium and ending with radon. Lead is currently the last stable element; all subsequent elements are radioactive. For bismuth, however, its only primordial isotope, 209Bi, has a half-life of more than 1019 years, over a billion times longer than the current age of the universe. As a rule, period 6 elements fill their 6s shells first, then their 4f, 5d, and 6p shells, in that order; however, there are exceptions, such as gold.
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 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.
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 usually used with internal combustion engines fueled by gasoline or diesel, including lean-burn engines, and sometimes on kerosene heaters and stoves.
The platinum-group metals are six noble, precious metallic elements clustered together in the periodic table. These elements are all transition metals in the d-block.
Group 7, numbered by IUPAC nomenclature, is a group of elements in the periodic table. They are manganese (Mn), technetium (Tc), rhenium (Re), and bohrium (Bh). All known elements of group 7 are transition metals.
The synthesis of precious metals involves the use of either nuclear reactors or particle accelerators to produce these elements.
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 trihydrated (n = 3) salt is widely used to prepare compounds used in homogeneous catalysis, notably for the industrial production of acetic acid and hydroformylation.
Nanomaterial-based catalysts are usually heterogeneous catalysts broken up into metal nanoparticles in order to enhance the catalytic process. Metal nanoparticles have high surface area, which can increase catalytic activity. Nanoparticle catalysts can be easily separated and recycled. They are typically used under mild conditions to prevent decomposition of the nanoparticles.
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.
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.
Rhodium-platinum oxide , or Nishimura's catalyst, is an inorganic compound used as a hydrogenation catalyst.
In organometallic chemistry, the activation of cyclopropanes by transition metals is a research theme with implications for organic synthesis and homogeneous catalysis. Being highly strained, cyclopropanes are prone to oxidative addition to transition metal complexes. The resulting metallacycles are susceptible to a variety of reactions. These reactions are rare examples of C-C bond activation. The rarity of C-C activation processes has been attributed to Steric effects that protect C-C bonds. Furthermore, the directionality of C-C bonds as compared to C-H bonds makes orbital interaction with transition metals less favorable. Thermodynamically, C-C bond activation is more favored than C-H bond activation as the strength of a typical C-C bond is around 90 kcal per mole while the strength of a typical unactivated C-H bond is around 104 kcal per mole.