1s² 2s²p⁶ 3s²p⁶d¹⁰ 4s²p⁶d⁸ 5s¹

History

Rhodium (Greek rhodon meaning "Rose") was discovered in 1803 by William Hyde Wollaston, soon after his discovery of palladium They made this discovery in England using crude platinum ore that they presumably obtained from South America.

Their procedure involved dissolving the ore in aqua regia, neutralizing the acid with sodium hydroxide, NaOH.  They then precipitated the platinum metal by adding ammonium chloride, NH₄Cl, as ammonium chloroplatinate.  The element palladium was removed as palladium cyanide after treating the solution with mercuric cyanide.  The material that remained was a red  sodium rhodium (III) chloride, Na₃RhCl₆·12H₂O: rhodium metal was isolated via reduction with hydrogen gas, H₂.

Characteristics

Rhodium is a hard silvery white and durable metal that has a high reflectance.  Rhodium metal does not normally form an oxide, even when heated.   Oxygen is absorbed from the atmosphere at the melting point of rhodium, but on solidification the oxygen is released.  Rhodium has both a higher melting point and lower density than platinum.  It is not attacked by acids: it is completely insoluble in nitric acid and slightly dissolves in aqua regia.  A complete dissolution of rhodium in powder form is only obtained in sulfuric acid.

Occurrence

The industrial extraction of rhodium is complex as the metal occurs in ores mixed with other metals such as palladium, silver, platinum, and gold.  It is found in platinum ores and obtained free as a white inert metal which it is very difficult to fuse. Principal sources of this element are located in South Africa, in river sands of the Ural Mountains, in North and South America and also in the copper-nickel sulfide mining area of the Sudbury, Ontario, Canada region.  Although the quantity at Sudbury is very small, the large amount of nickel ore processed makes rhodium recovery cost effective.  The main exporter of rhodium is South Africa (>80%) followed by Russia.  However, the annual world production of this element is only about 20 tons and there are very few rhodium minerals.  However, it is generally difficult to determine if a rock sample does or does not contain platinum group elements.  As of 2006, rhodium cost approximately six times as much as gold, by weight.

Applications

The primary use of this element is as an alloying agent for hardening 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:

• It is used as an electrical contact material due to its low electrical resistance, low and stable contact resistance, and its high corrosion resistance.
• Plated rhodium, made by electroplating or evaporation, is extremely hard and is used for optical instruments.
• This metal finds use in jewelry and for decorations.  It is electroplated on white gold and platinum to give it a reflective white surface.  This is known as rhodium flashing in the jewelry business.
• It is also a highly useful catalyst in a number of industrial processes (notably it is used in the catalytic system of automobile catalytic converters and for catalytic carbonylation of methanol to produce acetic acid by the Monsanto process).  It is used to catalyse addition of hydrosilanes to a double bond, a process important in manufacture of certain silicone rubbers.
• The complex of a rhodium ion with BINAP gives a widely used chiral catalyst for chiral synthesis, as in the synthesis of menthol.
• It is also used as a filter in mammography systems because of the characteristic x-rays it produces.

Symbolic Uses

Rhodium has been used for honors, or to symbolize wealth, when more commonly used metals such as silver, gold, or platinum are 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.  Guinness has also noted items such as the world's "Most Expensive Pen" or "Most Expensive Board Game" as containing rhodium.

Isotopes

Naturally occurring rhodium is composed of only one isotope, ¹⁰³Rh.   The most stable radioisotopes are ¹⁰¹Rh with a half-life of 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. Thirty-three radioisotopes have been characterized with atomic weights ranging from 88.948 u (⁹³Rh) to 121.94 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 207 days 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.

Precautions

Rhodium metal is, as a noble metal, inert.

However, when rhodium is chemically bound, it is reactive.  Rhodium compounds are not often encountered by most people and should be considered to be highly toxic and carcinogenic.  Lethal intake (LD50) for rats is 12.6 mg/kg of rhodium chloride (RhCl3).  Rhodium compounds can stain human skin very strongly.   This element plays no biological role in humans.

It is also possible to extract rhodium from spent nuclear fuel, which contains an average of 400 g of rhodium per metric ton. Rhodium produced in such a way contains radioactive isotopes with half-lives of up to 2.9 years and is therefore stored at least 20 years in a secured area to allow it to become stable.  Every 2.9 years of isolation reduces the radioactivity by 50%. Fissioned rhodium has a specific activity of 8.1 curies of radioactivity per gram after 5 years isolation.  Under health physics safety rules, any isotope that emits more than 1 curie of activity is a hazard; however after 8 years the activity falls to 4.1 Ci, after 11 years it is 2.2, after 14 years 1.1 Ci, after 17 years .55 Ci, and after 20 years only .27 Ci.  After 30 years the activity falls to 2.702E-4 Ci, which is under the threshold for low level risk by even the most stringent health physics rules.  The radioactivity of rhodium falls off so quickly because the percentage of Rh-102 in the recovered rhodium is only a trace amount and the remainder of the material absorbs the energy released.  Rhodium averaged $26.43 per gram or $820 per troy ounce over the last 30 years and $6200 per troy ounce in May 2006, adding significantly to the resource value of reprocessed fission waste.