/roh"dee euhm/, n. Chem.
a silvery-white metallic element of the platinum family, forming salts that give rose-colored solutions: used to electroplate microscopes and instrument parts to prevent corrosion. Symbol: Rh; at. wt.: 102.905; at. no.: 45; sp. gr.: 12.5 at 20°C.
[1804; < NL; see RHOD-, -IUM]

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 chemical element, one of the platinum metals (platinum group) of Groups 8–10 (VIIIb), Periods 5 and 6, of the periodic table, predominantly used as an alloying agent to harden platinum. Rhodium is a precious, silver-white metal, with a high reflectivity for light. It is not corroded or tarnished by the atmosphere at room temperature and is frequently electroplated onto metal objects and polished to give permanent, attractive surfaces for jewelry and other decorative articles. The metal is also used to produce reflecting surfaces for optical instruments.

      Rhodium added to platinum in small amounts yields alloys that are harder and lose weight at high temperatures even more slowly than pure platinum. Such alloys are used for laboratory furnace crucibles, spark-plug electrodes, and catalysts in very hot chemical environments (including automobile catalytic converters). In the industrial manufacture of nitric acid, gauze catalysts of rhodium–platinum alloys are used because they can withstand the flame temperature as ammonia is burned to nitric oxide. A wire of the alloy 10 percent rhodium–90 percent platinum joined to a wire of pure platinum forms an excellent thermocouple for measuring high temperatures in an oxidizing atmosphere. The international temperature scale is defined over the region from 660° to 1,063° C (1,220° to 1,945° F) by the electromotive force of this thermocouple.

      Rhodium is a rare element comprising up to 4.6 percent of native platinum alloys. It also occurs in native alloys of iridium and osmium: up to at least 11.25 percent in iridosmine and up to at least 4.5 percent in siserskite. Rhodium occurs in nature in association with the other platinum metals, and its separation and refinement form part of the overall metallurgical processing of the group. Rhodium is generally obtained commercially as a by-product of the extraction of nickel and copper from their ores.

      Natural rhodium consists entirely of stable isotope rhodium-103. The element was first isolated (1803) from crude platinum by the English chemist and physicist William Hyde Wollaston, who named it from the Greek rhodon (“rose”) for the red colour of a number of its compounds. Rhodium is highly resistant to attack by acids; the massive metal is not dissolved by hot concentrated nitric or hydrochloric acids or even by aqua regia. The metal dissolves in fused potassium hydrogen sulfate to yield a complex, water-soluble sulfate K3Rh(SO4)3·12H2O, in hot concentrated sulfuric acid, and in concentrated hydrochloric acid containing sodium perchlorate at 125°–150° C (257°–302° F).

      Rhodium chemistry centres chiefly on the +1 and +3 oxidation states; a few compounds of the other positive oxidation states through +6 are recognized. Rhodium forms the dirhodium tetraacetate, Rh2(O2CCH3)4 and various derivatives containing two additional ligands—e.g., water, pyridine, or triphenylphosphine—in oxidation state +2. Complexes in oxidation state +1 chiefly contain carbon monoxide, olefins, and phosphines as ligands. All rhodium compounds are readily reduced or decomposed by heating to yield the powdered or sponge metal. Among these compounds rhodium trichloride, RhCl3 (in which rhodium is in the +3 state), is one of the most important. It provides a starting material for many of the other rhodium compounds in various oxidation states. In aqueous emulsions it can catalyze a number of useful organic reactions.

atomic number
atomic weight
melting point
1,966° C (3,571° F)
boiling point
3,727° C (6,741° F)
specific gravity
12.4 (20° C)
oxidation states
+1, +2, +3, +4, +5, +6
electronic config.

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Universalium. 2010.

Look at other dictionaries:

  • rhodium — [ rɔdjɔm ] n. m. • 1805; de rhod(o) et ium, à cause de la couleur rose de ses dérivés ♦ Chim. Élément atomique (Rh; no at. 45; m. at. 102,90), métal de transition très dur, extrait des minerais de platine et d or avec lesquels il forme des… …   Encyclopédie Universelle

  • Rhodium — Ruthénium ← Rhodium → Palladium Co …   Wikipédia en Français

  • Rhodium — Rho di*um, n. [NL., fr. Gr. ro don the rose. So called from the rose red color of certain of its solutions. See {Rhododendron}.] (Chem.) A rare element of the light platinum group. It is found in platinum ores, and obtained free as a white inert… …   The Collaborative International Dictionary of English

  • Rhodĭum — Rhodĭum, Rh, Atomgewicht = 650 (O = 100), 52 (H = 1), ein von Wollaston 1803 im rohen Platin entdecktes seltenes Metall. Man gewinnt es, indem man aus der Lösung der Platinerze Palladium u. den größten Theil des Platins entfernt, u. aus der… …   Pierer's Universal-Lexikon

  • Rhodĭum — Rh, eins der Platinmetalle, findet sich besonders im Osmiumiridium, mit Gold legiert in Mexiko und wird aus den Platinrückständen gewonnen. Es ist grauweiß, strengflüssiger als Platin, sehr dehnbar und hämmerbar, spez. Gew. 12,1, Atomgew. 103,… …   Meyers Großes Konversations-Lexikon

  • Rhodium — Rhodium, Rh (Atomgewicht 103, spez. Gew. 12,6, Schmelzpunkt gegen 1700°), grauweißes, hämmer und schweißbares Metall der Platingruppe, in den russischen Platinerzen zu 2–3% enthalten. In Säuren und Königswasser unlöslich. Es findet, mit… …   Lexikon der gesamten Technik

  • Rhodium — Rhodĭum (chem. Zeichen Rh), in den Platinerzen vorkommendes Metall, fast silberweiß, schwerer schmelzbar als Platin, spez. Gewicht 12,2, Atomgewicht 103; seine Salze sind rosenrot gefärbt; zu Goldfederspitzen, in Legierung mit Platin zu… …   Kleines Konversations-Lexikon

  • Rhodium — Rhodium, 1804 von Wollaston in den Platinerzen entdecktes Metall, rein silberweiß, hart, spröd, fast unschmelzbar, spec. Gew. 11; seine Oxyde sind z. Theil rosenroth, daher der Name R …   Herders Conversations-Lexikon

  • rhodium — Symbol: Rh Atomic number: 45 Atomic weight: 102.905 Silvery white metallic transition element. Found with platinum and used in some platinum alloys. Not attacked by acids, dissolves only in aqua regia. Discovered in 1803 by W.H. Wollaston …   Elements of periodic system

  • rhodium — (n.) hard white metallic element, 1804, named for the color of solutions containing it, from Gk. rhodon rose (see ROSE (Cf. rose) (n.)) + metallic element ending ium …   Etymology dictionary

  • rhodium — [rō′dē əm] n. [ModL: so named (1804) by its discoverer, W. H. Wollaston (see WOLLASTONITE) < Gr rhodon, rose, after the color of a dilute solution of its salts + IUM] a hard, gray white metallic chemical element, one of the platinum metals,… …   English World dictionary

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