Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Palladium is found in Row 5, Group 10 (VIIIB) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. Palladium, ruthenium, rhodium, osmium, indium, and platinum make up the platinum group of metals. These metals are also sometimes called the noble metals. That term reflects the fact that the six elements are not very reactive.
Like the other platinum metals, palladium is quite rare. It also has a beautiful shiny finish that does not tarnish easily. These properties make it desirable in making jewelry and art objects. These uses are its most important applications.
Group 10 (VIIIB)
Discovery and naming
As early as the 1700s, Brazilian miners described a number of platinum-like metals. They called them by such names as prata (silver), ouro podre (worthless or spoiled gold), and ouro bran-cowhite gold). These names may or may not have matched the materials that were actually present in the metals. For example, a substance the miners called platino (platinum) was probably a combination of gold and palladium.
In the early 1800s, Wollaston received samples of some of these metals. He decided to analyze them. During his work, he found that a sample of platinum ore contained other metals as well. These metals turned out to be two new elements—rhodium and palladium. The name palladium was taken from Pallas, an asteroid that had been discovered at about the same time.
Chemists later found palladium in other South American ores. A sample of ouro podre, for example, turned out to include about 86 percent gold, 10 percent palladium, and 4 percent silver.
Palladium is a soft, silver-white metal. It is both malleable and ductile. Malleable means capable of being hammered into thin sheets. Ductile means capable of being drawn into thin wires. The malleability of palladium is similar to that of gold. It can be hammered into sheets no more than about a millionth of a centimeter thick.
An interesting property of palladium is its ability to absorb (soak up) hydrogen gas like a sponge. When a surface is coated with finely divided palladium metal, the hydrogen gas passes into the space between palladium atoms. Palladium absorbs up to 900 times its own weight in hydrogen gas.
Palladium has been called "the least noble" of the noble metals because it is the most reactive of the platinum group. It combines poorly with oxygen under normal conditions but will catch fire if ground into powder. Palladium does not react with most acids at room temperature but will do so when mixed with most hot acids. The metal will also combine with fluorine and chlorine when very hot.
Occurrence in nature
The abundance of palladium in the Earth's crust is estimated to be about 1 to 10 parts per trillion. That makes it one of the ten rarest elements found in the Earth's crust. It usually occurs in native form, meaning "not combined with any other element." Palladium is usually found with platinum and other members of the noble metal group.
Palladium combines poorly with oxygen under normal conditions but will catch fire if ground into powder.
There are six naturally occurring isotopes of palladium: palladium-102, palladium-105, palladium-106, palladium-108, and palladium-110. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
About a dozen radioactive isotopes of palladium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
No isotope of palladium has an important commercial use.
Palladium is removed from platinum ores after platinum and gold have been removed. The metal is converted to palladium chloride (PdCl2) and then purified as pure palladium.
Palladium has two primary uses: as a catalyst and in making jewelry and specialized alloys. A catalyst is a substance used to speed up or slow down a chemical reaction without undergoing any change itself. Palladium catalysts are used in breaking down petroleum to make high quality gasoline and other products. It is also used in the production of some essential chemicals, such as sulfuric acid (H2SO4), which is used in paper and fabric production. The catalytic converters used in automobiles today may also contain a palladium catalyst. A catalytic converter is a device added to a car's exhaust system. It helps the fuel used in the car burn more efficiently.
An alloy is made by mixing two or more melted metals. The solid mixture has properties different from those of the individual metals. Palladium is commonly alloyed with gold, silver, and copper. The alloys are used in a variety of products, such as ball bearings, springs, balance wheels of watches, surgical instruments, electrical contacts, and astronomical mirrors. Palladium alloys are also used quite widely in dentistry.
Relatively few palladium compounds are commercially important.
Many automobiles use a palladium catalyst to help fuel burn more efficiently.
There is no evidence of serious health effects from exposure to palladium or its compounds.
The element palladium was isolated and identified by William Wollaston in 1803. Its name comes from the asteroid Pallas. (Pallas was another name for Athena, the Greek goddess of wisdom.) Palladium in pure form is not found in nature. The preparation of the element is via a series of reactions. Platinum metal ore concentrates (65% of which come from the Merensky Reef in South Africa) are treated with aqua regia (giving copper and nickel as byproducts). The solutions, containing H2PdCl4 with platinum and gold complexes, are treated with FeCl2 (which precipitates gold) and then with excess of NH4OH followed by HCl to precipitate the impure [Pd(NH3)2Cl2]. This compound is purified by dissolution in NH4OH and precipitation with HCl. The pure [Pd(NH3)2Cl2] is ignited to palladium metal.
Palladium metal, like platinum metal, is silvery-white and lustrous and has malleable and ductile properties. It has the face-centered cubic crystal structure. It forms a fluoride, PdF4 (brick-red), and other halides: PdF2 (pale violet), α -PdCl2 (dark red), PbBr2 (red black), and PdI2 (black). Pd metal can absorb up to 935 times its own volume of hydrogen molecules. When the composition reaches about PdH0.5, the substance becomes a semiconductor.
Palladium can form complexes in a variety of oxidation states. Table 1 contains some examples.
Palladium has extensive use as a catalyst in hydrogenation and dehydrogenation reactions, due to its capacity of combination with hydrogen. Palladium films are used as electrical contacts in connectors. Palladium-silver and palladium-nickel alloys are used to substitute for gold in jewelry.
|(IV)||K2[PdF6] (bright yellow)|
Lea B. Zinner
Allred, A. L. (1961). Journal of Inorganic Nuclear Chemistry 17:215. Greenwood, Norman N., and Earnshaw, A. (1984). Chemistry of the Elements. New York: Pergamon Press.
Livingstone, Stanley E. (1973). "The Platinum Metals." In Comprehensive Inorganic Chemistry, Vol. 3, ed. J. C. Bailar Jr.; H. J. Emeléus; Sir Ronald Nyholm; and A.F. Trotman-Dickenson. Oxford, U.K.: Pergamon Press.
pal·la·di·um1 / pəˈlādēəm/ • n. the chemical element of atomic number 46, a rare silvery-white metal resembling platinum. (Symbol: Pd) pal·la·di·um2 • n. (pl. -di·a / -dēə/ ) archaic a safeguard or source of protection.