Jan 31 2003
Rhodium (Rh) was discovered in 1803 by W.H. Woolaston in London.
One of the rarest metals on earth (abundance of 2 x 10-4 ppm), Rhodium does not appear naturally, tending to be found with other platinum group metals.
It is a hard, lustrous, silvery coloured metal which is stable in air. Rhodium is inert to all acids but is attacked by fused alkalis.
The metal has high thermal and electrical conductivities and is alloyed with platinum to form the positive wire of a Pt/Rh - Pt thermocouple.
Other applications of Rhodium include its use as a plating material (to provide a hard and bright surface which is resistant to oxidation), as a catalyst and also as an alloying element, where it improves the hardness of the resulting alloy.
Typical Properties
The key properties of Rhodium are tabulated below.
Table 1. Key properties
| Atomic Properties |
| Atomic number |
45 |
| Atomic radius - Goldschmidt ( nm ) |
0.134 |
| Atomic weight ( amu ) |
102.9055 |
| Crystal structure |
Face centred cubic |
| Electronic structure |
Kr 4d8 5s1 |
| Ionisation potential |
No. |
eV |
| |
1 |
7.46 |
| |
2 |
18.1 |
| |
3 |
31.1 |
| Natural isotope distribution |
Mass No. |
% |
| |
103 |
100 |
| Photo-electric work function ( eV ) |
4.6 |
| Thermal neutron absorption cross-section ( Barns ) |
150 |
| Valences shown |
2, 3, 4, 5, 6 |
| Electrical Properties |
| Electrical resistivity @ 20 C ( µOhm.cm ) |
4.7 |
| Temperature coefficient @ 0-100 C ( K-1 ) |
0.0044 |
| Thermal emf against Pt (cold 0 °C - hot 100 °C) ( mV ) |
+0.70 |
| Mechanical Properties |
| Material condition |
Soft |
Hard |
Polycrystalline |
| Bulk modulus ( GPa ) |
|
|
276 |
| Hardness – Vickers |
120 |
300 |
|
| Poisson's ratio |
|
|
0.26 |
| Tensile modulus ( GPa ) |
|
|
379 |
| Tensile strength ( MPa ) |
690-760 |
1380-2070 |
|
| Yield strength ( MPa ) |
69-275 |
|
|
| Physical Properties |
| Boiling point (°C ) |
3727 |
| Density @ 20 C ( g.cm-3 ) |
12.4 |
| Melting point (°C ) |
1965 |
| Thermal Properties |
| Coefficient of thermal expansion @ 0-100 C ( x10-6 K-1 ) |
8.5 |
| Latent heat of evaporation ( J.g-1 ) |
4800 |
| Latent heat of fusion ( J.g-1 ) |
210 |
| Specific heat @ 25 C ( J.K-1.kg-1 ) |
244 |
| Thermal conductivity @ 0-100 C ( W.m-1.K-1 ) |
150 |
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