1s² 2s²p⁶ 3s²p⁶d¹⁰ 4s²p⁶d¹⁰

History

Palladium was discovered by William Hyde Wollaston in 1803.  This element was named by Wollaston in 1804 after the asteroid Pallas, which was discovered two years earlier.

Wollaston found palladium in crude platinum ore from South America by dissolving the ore in aqua regia, neutralizing the solution with sodium hydroxide, and precipitating platinum as ammonium chloroplatinate with ammonium chloride.  He added mercuric cyanide to form the compound palladium cyanide, which was heated to extract palladium metal.

Palladium chloride was at one time prescribed as a tuberculosis treatment at the rate of 0.065g per day (approximately one milligram per kilogram of body weight).  This treatment did not have many negative side effects, but was later replaced by more effective drugs.

Palladium's affinity for hydrogen led it to play an essential role in the Fleischmann-Pons experiment, also known as cold fusion.

In 2000, Ford Motor Company created a price bubble in palladium by stockpiling large amounts of the metal, fearing interrupted supplies from Russia.  As prices fell in early 2001, Ford lost nearly $1 billion dollars.

The demand of palladium increased from 100 tons in 1990 to nearly 300 tons in 2000, most of the palladium is used for catalytic converters in the automobile industry.

Characteristics

Palladium is a soft silver-white metal that resembles platinum.  It is the least dense and has the lowest melting point of the platinum group metals.  It is soft and ductile when annealed and greatly increases its strength and hardness when it is cold-worked.  Palladium is chemically attacked by sulfuric, nitric and hydrochloric acid in which it dissolves slowly.  This metal also does not react with oxygen at normal temperatures (and thus does not tarnish in air).  Palladium heated to 800°C will produce a layer of palladium (II) oxide (PdO).  It lightly tarnishes in moist atmosphere containing sulfur.

This metal has the uncommon ability to absorb up to 900 times its own volume of hydrogen at room temperatures.  It is thought that this possibly forms palladium hydride (PdH₂) but it is not yet clear if this is a true chemical compound.

When palladium has absorbed large amounts of hydrogen, it can swell up, like a sponge full of water, visible to the naked eye.

Common oxidation states of palladium are 0,+1, +2 and +4.  Although originally +3 was thought of as one of the fundamental oxidation states of palladium, there is no evidence for palladium occurring in the +3 oxidation state; this has been investigated via X-ray diffraction for a number of compounds, indicating a dimer of palladium (II) and palladium (IV) instead.  Recently, compounds with an oxidation state of +6 were synthesized.

Occurrence

Palladium is found as a free metal and alloyed with platinum and gold with platinum group metals in placer deposits of the Ural Mountains, Australia, Ethiopia, South and North America.  It is commercially produced from nickel-copper deposits found in South Africa; Ontario, Canada and Siberia; the huge volume of ore processed makes this extraction profitable in spite of the low proportion of palladium in these ores.  The world's largest single producer of Palladium is MMC Norilsk Nickel, headquartered in Moscow, Russia.

Applications

Palladium is also used in dentistry, watch making, in aircraft spark plugs and in the production of surgical intruments and electrical contacts.  Palladium is also used to make professional transverse flutes.

Technology

Hydrogen easily diffuses through heated palladium; thus, it provides a means of purifying the gas.  Palladium (and palladium-silver alloys) are used as electrodes in multi-layer ceramic capacitors.  Palladium (sometimes alloyed with nickel is used in connector platings in consumer electronics.

It is also used as Palladium-Hydrogen electrode in electrochemical studies.   Palladium dichloride (PdCl₂), a palladium compound, can absorb large amounts of carbon monoxide (CO) gas and is used in carbon monoxide detectors.

Catalysis

When it is finely divided, such as in palladium on carbon, palladium forms a good catalyst and is used to speed up hydrogenation and dehydrogenation reactions, as well as in petroleum cracking.  A large number of carbon-carbon bond forming reactions in organic chemistry (such as the Heck and Suzuki coupling) are facilitated by catalysis with palladium compounds.  The largest use of palladium today is in catalytic converters.   Much research is in progress to discover ways to replace the much more expensive platinum with palladium in this application.

Jewelry

Since 1939 palladium itself has occasionally been used as a precious metal in jewelry, as replacement for platinum or white gold.  This is due to its naturally white properties giving it no need for a rhodium plating.  It's slightly whiter, much lighter, about 12% harder, and currently much cheaper than platinum and it's more hypoallergenic, more "natural" than white gold.  Similar to gold, palladium can be beaten into a thin leaf form as thin as 100 nm (1/250,000 in).  Like platinum, it will develop a hazy patina over time.  Unlike platinum, palladium will discolor at soldering temperatures, become brittle with repeated heating and cooling, and react with strong acids however.

It can also be used as a substitute for nickel when making white gold.  Palladium is one of three most used metals which can be alloyed with gold to produce white gold (nickel and silver can also be used).  Palladium-gold is a much more expensive alloy than nickel-gold but is hypoallergenic and holds its white color better.

When platinum was declared a strategic government resource during World War II,   many jewelry bands were made out of palladium.  As recently as 2001, palladium was more expensive than platinum and rarely used in jewelry.  Its use in jewelry has increased because of a large spike in the price of platinum.  Respected jewelry maker Scott Kay, who formerly only used platinum in his jewelry lines, has recently been one of the most vocal advocates for palladium in jewelry.

Photography

With the platinotype printing process photographers make fine-art black-and-white prints using platinum or palladium salts. Often used with platinum, palladium provides an alternative to silver.

Compounds

Isotopes

Naturally-occurring palladium is composed of six isotopes.  The most stable radioisotopes are ¹⁰⁷Pd with a half-life of 6.5 million years, ¹⁰³Pd with a half-life of 17 days, and ¹⁰⁰Pd with a half-life of 3.63 days.  Twenty-eight other radioisotopes have been characterized with atomic weights ranging from 90.949 u (⁹¹Pd) to 123.936 u (¹²⁴Pd).  Most of these have half-lives that are less than a half an hour except ¹⁰¹Pd (half-life: 8.47 hours), ¹⁰⁹Pd (half-life: 13.7 hours), and ¹¹²Pd (half-life: 21 hours).

The primary decay mode before the most abundant stable isotope, ¹⁰⁶Pd, is electron capture and the primary mode after is beta decay.  The primary decay product before ¹⁰⁶Pd is rhodium and the primary product after is silver.

Radiogenic ¹⁰⁷Ag is a decay product of ¹⁰⁷Pd and was first discovered in the Santa Clara, California meteorite of 1978.  The discoverers suggest that the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after a nucleosynthetic event.  ¹⁰⁷Pd versus Ag correlations observed in bodies, which have clearly been melted since accretion of the solar system, must reflect the presence of short-lived nuclides in the early solar system.