palladium (Pd),chemical element, the least dense and lowest-melting of theplatinum metals of Groups 8–10 (VIIIb), Periods 5 and 6, of theperiodic table, used especially as acatalyst (a substance that speeds upchemical reactions without changing their products) and inalloys.

Aprecious gray-whitemetal, palladium is extremely ductile and easily worked. Palladium is not tarnished by theatmosphere at ordinarytemperatures. Thus, the metal and its alloys serve as substitutes forplatinum injewelry and in electrical contacts; the beaten leaf is used for decorative purposes. Relatively small amounts of palladium alloyed withgold yield the best white gold. Palladium is used also in dental alloys. The chief use of palladium, however, is inautomobilecatalytic converters (often in combination withrhodium); the palladium serves as acatalyst to convert pollutinghydrocarbons,carbon monoxide, and nitrogen oxide in the exhaust towater,carbon dioxide, andnitrogen. Palladium coatings, electrodeposited or chemically plated, have been used in printed-circuit components, and palladium is also used in multilayerceramiccapacitors.

Native palladium, though rare, occurs alloyed with a little platinum andiridium in Colombia (department of Chocó), in Brazil (Itabira, Minas Gerais), in theUral Mountains, and inSouth Africa (theTransvaal). Palladium is one of the most abundant platinum metals and occurs inEarth’s crust at an abundance of 0.015 part per million. For the mineralogical properties of palladium,seenative element (table). Palladium also occurs alloyed with native platinum. It was first isolated (1803) from crude platinum by the English chemist and physicistWilliam Hyde Wollaston. He named the element in honor of the newly discoveredasteroidPallas. Palladium is also associated with a number of gold,silver,copper andnickel ores. It is generally produced commercially as a by-product in the refining of copper and nickel ores. Russia, South Africa, Canada, and theUnited States were the world’s leading producers of palladium in the early 21st century.

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Surfaces of palladium are excellentcatalysts for chemical reactions involvinghydrogen andoxygen, such as thehydrogenation of unsaturatedorganic compounds. Under suitable conditions (80 °C [176 °F] and 1 atmosphere), palladium absorbs more than 900 times its own volume of hydrogen. It expands and becomes harder, stronger, and less ductile in the process. The absorption also causes both the electrical conductivity andmagnetic susceptibility to decrease. A metallic or alloylike hydride is formed from which the hydrogen can be removed by increased temperature and reduced pressure. Because hydrogen passes rapidly through the metal at high temperatures, heated palladium tubesimpervious to other gases function as semipermeable membranes and are used to pass hydrogen in and out of closed gas systems or for hydrogen purification.

Palladium is more reactive than the other platinum metals. For example, it is attacked more readily byacids than any of the other platinum metals. It dissolves slowly innitric acid to give palladium(II) nitrate, Pd(NO₃)₂, and with concentratedsulfuric acid it yields palladium(II) sulfate, PdSO₄∙2H₂O. In its sponge form it will dissolve even inhydrochloric acid in the presence ofchlorine or oxygen. It is rapidly attacked by fused alkali oxides and peroxides and also byfluorine and chlorine at about 500 °C (932 °F). Palladium also combines with a number of nonmetallic elements on heating, such asphosphorus,arsenic,antimony,silicon,sulfur, andselenium.

A series of palladiumcompounds can be prepared with the +2 oxidation state; numerouscompounds in the +4 state and a few in the 0 state are also known. Among the transition metals palladium has one of the strongest tendencies to form bonds withcarbon. All palladium compounds are easily decomposed or reduced to the free metal. An aqueous solution ofpotassium tetrachloropalladate(II), K₂PdCl₄, serves as a sensitive detector for carbon monoxide orolefin gases because a black precipitate of the metal appears in the presence of exceedingly small amounts of those gases. Natural palladium consists of a mixture of six stableisotopes: palladium-102 (1.02 percent), palladium-104 (11.14 percent), palladium-105 (22.33 percent), palladium-106 (27.33 percent), palladium-108 (26.46 percent), and palladium-110 (11.72 percent).

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