Rhodium

ruthenium – rhodium – palladium Co Rh Ir    Full table
General
Name, Symbol, Number	rhodium, Rh, 45
Chemical series	Transition metals
Group, Period, Block	9, 5, d
Density, Hardness	12450 kg/m3, 6
Appearance	Silvery white metallic
Atomic properties
Atomic weight	102.90550 u
Atomic radius (calc.)	135 (173) pm
Covalent radius	135 pm
van der Waals radius	no data
Electron configuration	[Kr]4d8 5s1
e- 's per energy level	2, 8, 18, 16, 1
Oxidation states (Oxide)	2, 3, 4 (amphoteric)
Crystal structure	Face centered cubic
Physical properties
State of matter	Solid (__)
Melting point	2237 K (3567 ?F)
Boiling point	3968 K (6683 ?F)
Molar volume	8.28 ×10-6 m3/mol
Heat of vaporization	493 kJ/mol
Heat of fusion	21.5 kJ/mol
Vapor pressure	0.633 Pa at 2239 K
Speed of sound	4700 m/s at 293.15 K
Miscellaneous
Electronegativity	2.28 (Pauling scale)
Specific heat capacity	0.242 J/(kg*K)
Electrical conductivity	21.1 106/(m?ohm)
Thermal conductivity	150 W/(m*K)
1st ionization potential	719.7 kJ/mol
2nd ionization potential	1740 kJ/mol
3rd ionization potential	2997 kJ/mol
Most stable isotopes
iso NA half-life DM DE MeV DP 101Rh {syn.} 3.3 y ε 0.542 101Ru 102Rhm {syn.} ~2.9 y ε IT 2.464 0.141 102Ru   103Rh 100% Rh is stable with 58 neutrons
SI units & STP are used except where noted.

Rhodium is a chemical element in the periodic table that has the symbol Rh and atomic number 45. A rare silvery-white hard transition metal, rhodium is a member of the platinum group, is found in platinum ores and is used in alloys with platinum and as a catalyst.

Contents

1 Notable characteristics 2 Applications 3 History 4 Occurrence 5 Isotopes 6 Precautions 7 References

Notable characteristics

Rhodium is a hard silvery white and durable metal that has a high reflectance. It changes in air to the sesquioxide while slowly cooling from a red hot state but at higher temperatures converts back to the metal. Rhodium has both a higher melting point and lower density than platinum. It is not attacked by acids and only dissolves in aqua regia.

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;

• 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 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).

History

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

His procedure involved dissolving the ore in aqua regia, neutralizing the acid with sodium hydroxide (NaOH). He 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 rhodium(III) chloride: rhodium metal was isolated via reduction with hydrogen gas.

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 river sands of the Ural Mountains, in North and South America and also in the copper-nickel sulfide mining area of the Sudbury, Ontario region. Although the quantity at Sudbury is very small, the large amount of nickel ore processed makes rhodium recovery cost effective. However, the annual world production of this element is only 7 or 8 tons and there are very few rhodium minerals.

It is also possible to extract Rhodium from burned-out nuclear fuel, which contains a few percent of rhodium. Rhodium produced in such a way contains radioactive isotopes with half-lives of up to 45 days and it must be checked very carefully for radioactivity before being sold.

Isotopes

Naturally occurring rhodium is composed of only one isotope (Rh-103). The most stable radioisotopes are Rh-101 with a half-life of 3.3 years, Rh-102 with a half-life of 207 days, and Rh-99 with a half-life of 16.1 days. Twenty other radioisotopes have been characterized with atomic weights ranging from 92.926 u (Rh-93) to 116.925 u (Rh-117). Most of these have half-lifes that are less than an hour except Rh-100 (half-life: 20.8 hours) and Rh-105 (half-life: 35.36 hours). There are also numerous meta states with the most stable being Rhm-102 (0.141 MeV) with a half-life of about 2.9 years and Rhm-101 (0.157 MeV) with a half-life of 4.34 days.

The primary decay mode before the only stable isotope, Rh-103, is electron capture and the primary mode after is beta emission. The primary decay product before Rh-103 is ruthenium and the primary product after is palladium.

Precautions

Compounds that contain rhodium are not often encountered by most people and should be considered to be highly toxic and carcinogenic. Rhodium compounds can stain human skin very strongly. This element plays no biological role in humans. Template:Chem clipart

References

• Los Alamos National Laboratory – Rhodium (http://periodic.lanl.gov/elements/45.html)