Rhodium
Symbol |
Name |
Atomic Number |
Atomic Weight |
Group Number |
Rh |
Rhodium |
45 |
102.91 |
9 |
Standard Sate: solid at 298 K
Color: silvery white metallic
Rhodium metal is silvery white. Rhodium has a higher melting point and lower density than platinum. It has a high reflectance and is hard and durable. Upon heating it turns to the oxide when red and at higher temperatures turns back to the element. It is a major component of industrial catalytic systems such as the BP-Monsanto process.
(Rh), chemical element, one of the platinum metals of Group VIII 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. Crucibles for heating materials in the chemical
laboratory are therefore commonly made from these alloys. In the
industrial manufacture of nitric acid, gauze catalysts of the
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 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)312H2O or in hot concentrated sulfuric acid.
Rhodium chemistry centres chiefly on the valence states +1 and +3;
a few compounds of the other positive states through +6 are
recognized (with perhaps the exception of +2). All rhodium
compounds are readily reduced or decomposed by heating to yield the
powdered or sponge metal. Among these compounds rhodium(III)
chloride, or rhodium trichloride, RhCl3, 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 45
atomic weight 102.905 melting point 1,966 C (3,571 F) boiling point
3,727 C (6,741 F) specific gravity 12.4 (20 C) valence 1, 2, 3, 4, 5, 6
electronic config. 2-8-18-16-1 or (Kr)4d85s1
"rhodium" Encyclop�dia Britannica Online.
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