Rhodium was discovered in 1803 and is member of the platinum group. It is widely used as a catalytic converter in automobile fuel industry that was introduced by Volvo in 1976.
History and Discovery
Rhodium was discovered by William Hyde Wollaston in 1803 [1]. He extracted the element from its platinum ore that was obtained from South Africa. The name rhodium has been derived from Greek word rhodon that means rose. The new element was named rose due to the colored compound formed when compound of rhodium with chloride was dissolved in aqua regia.
Rhodium
| Periodic Table Classification | Group 9 Period 5 |
|---|---|
| State at 20C | Solid |
| Color | Silvery white metallic |
| Electron Configuration | [Kr] 4d8 5s1 |
| Electron Number | 45 |
| Proton Number | 45 |
| Electron Shell | 2, 8, 18, 16, 1 |
| Density | 12.41 g.cm-3 at 20°C |
| Atomic number | 45 |
| Atomic Mass | 102.91 g.mol -1 |
| Electronegativity according to Pauling | 2.28 |
Occurrence
Rhodium is extremely rare metal and is present in around 0.0002 parts per million of the earth’s crust. The minerals and ores of rhodium are not very common in the earth’s crust. Rhodium is found in free form in nature, as well as alloyed with gold and other metals of the platinum group. Rhodium is also present in form of minerals such as rhodplumsite and bowieite. Commercially, rhodium is extracted from its nickel and platinum ores. It is also produced as a byproduct during the uranium-235 fission reaction, however, this extraction is complex and makes the commercial level production through this method fairly impossible. The largest producers of rhodium include South Africa, Canada and Russia, where large natural deposits of rhodium metal are present [2].
Physical Characteristics
Rhodium is silvery white transition metal. It has a shiny appearance and is hard in nature. Rhodium is a Nobel metal and belongs to the group of exquisite metals (including rhodium, platinum, osmium, ruthenium, osmium and iridium) termed as the platinum group metals (PGMs). Rhodium is a durable metal. It does not react with oxygen even at high temperatures. That is why, rhodium is highly resistant to corrosion and tarnishing. Its density is lower than platinum, around 12.41 g/cm3. Rhodium has a higher melting point as compared to platinum, 1964 °C and have a boling point of 3695 °C.
Chemical Characteristics
Rhodium is non-reactive, chemically inert element. It is not dissolved in dilute nitric acid, or aqua regia [3]. Rhodium does not react with oxygen and that is why is resistant to tarnishing in air. The most common oxidation state of rhodium is +3, but other oxidation states such as +0 and +6 also exist. Compounds of rhodium resemble compounds of platinum. It does not form volatile compounds. It reacts with halogens to form halides with various oxidation states. Most of the oxides of rhodium are stable.
Significance and Uses
- About 80% of rhodium that is extracted worldwide is used as a catalytic converter, for the catalytic conversion of harmful gases from automobile exhaust, carbon monoxide and hydrocarbon into less harmful gasses, such as nitrogen, carbon dioxide and water. Rhodium specifically reduced the amount of NOx in the automobile exhaust.
- Rhodium is widely used in making of various alloys to improve the hardness and corrosion resistance of other metals, such as platinum and palladium.
- Rhodium is also used to increase the beauty and shine of various precious metals, such as silver is lined with rhodium to make it resistant to tarnishing and discoloration and it is used as an additional attractive plating on gold to enhance its shine and appearance.
- Rhodium is used in nuclear power plants.
- Rhodium is used in glass industry, to produce flat panel glass and fiber glass.
- Rhodium is used in making ornaments and jewellery.
Health Hazards
In free form, rhodium is an inert and nontoxic metal. However, various compounds and salts f rhodium have moderate toxicity. Individuals working with rhodium are subjected to occupational hazards and the Occupational Safety and Health Administration (OSHA) has determined the permissible exposure dose of for rhodium in the workplace at 0.1 mg/m3 over an eight-hour work duration. In mice, the median lethal dose of soluble rhodium has been estimated to be around 198mg/ kg of body weight.
Isotopes of Rhodium
Rhodium has one naturally occurring isotope, rhodium-103. The most stable radio-isotope is rhodium-101 that has a half-life of around 3.3 years. There are twenty other radioactive isotopes of rhodium that are artificially produced and have atomic masses that range from 92.926 u to 116.925 u. most of these isotopes have half-life of less than an hour, expect rhodium-100 that has a half-life of 20.8 hours and rhodium-35.36 with a half-life of 35.36 hours [4].
REFERENCES
[1]. Hammond, C. R. (2004). “The Elements”. Handbook of Chemistry and Physics (81st ed.). CRC press. ISBN978-0-8493-0485-9.
[2]. “Platinum-Group Metals” (PDF). Mineral Commodity Summaries. United States Geological Survey. January 2007.
[3]. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 1113. ISBN 0-08-037941-9.
[4]. Audi, G.; Bersillon, O.; Blachot, J.; Wapstra, A. H. (2003). “The NUBASE Evaluation of Nuclear and Decay Properties”. Nuclear Physics A. Atomic Mass Data Center. 729: 3–128. Bibcode:2003NuPhA.729….3A. doi:10.1016/j.nuclphysa.2003.11.001.