Palladium belongs to group 10 in the periodic table, but the configuration in the outermost electron shells is atypical for group 10 (see also niobium (41), ruthenium (44), and rhodium (45)). Fewer electron shells are filled than the elements directly preceding it (a phenomenon unique to palladium). The valence shell has eighteen electrons – ten more than the eight found in the valence shells of the noble gases from neon onward.
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 is greatly increased in strength and hardness when cold-worked. Palladium dissolves slowly in concentrated nitric acid, in hot, concentrated sulfuric acid, and when finely ground, in hydrochloric acid. It dissolves readily at room temperature in aqua regia.
Palladium does not react with oxygen at standard temperatures (and thus does not tarnish in air). Palladium heated to 800 °C will produce a layer of palladium(II) oxide (PdO). It tarnishes lightly in a moist atmosphere containing sulfur.[clarification needed]
Palladium films with defects produced by alpha particle bombardment at low temperature exhibit superconductivity having Tc=3.2 K.
Naturally occurring palladium is composed of seven isotopes, six of which are stable. The most stable radioisotopes are 107Pdwith a half-life of 6.5 million years (found in nature), 103Pd with 17 days, and 100Pd with 3.63 days. Eighteen other radioisotopes have been characterized with atomic weights ranging from 90.94948(64) u (91Pd) to 122.93426(64) u (123Pd). These have half-lives of less than thirty minutes, except 101Pd (half-life: 8.47 hours), 109Pd (half-life: 13.7 hours), and 112Pd (half-life: 21 hours).
For isotopes with atomic mass unit values less than that of the most abundant stable isotope, 106Pd, the primary decay mode iselectron capture with the primary decay product being rhodium. The primary mode of decay for those isotopes of Pd with atomic mass greater than 106 is beta decay with the primary product of this decay being silver.
Radiogenic107Ag is a decay product of 107Pd and was first discovered in 1978 in the Santa Clara meteorite of 1976. 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 been melted since accretion of thesolar system, must reflect the presence of short-lived nuclides in the early solar system.
Palladium compounds primarily exist in the 0 and +2 oxidation state. Other less common states are also recognized. Generally the compounds of palladium are more similar to those of platinum than those of any other element.
Palladium(II) chloride is the principal starting material for other palladium compounds. It arises by the reaction of palladium with chlorine. It is used to prepare heterogeneous palladium catalysts such palladium on barium sulfate, palladium on carbon, and palladium chloride on carbon. Solutions of PdCl2 in nitric acid react with acetic acid to give palladium(II) acetate, also a versatile reagent. PdCl2 reacts with ligands (L) to give square planar complexes of the type PdCl2L2. One example of such complexes include the benzonitrile derivative PdX2(PhCN)2.
William Hyde Wollaston noted the discovery of a new noble metal in July 1802 in his lab-book and named it palladium in August of the same year. Wollaston purified enough of the material and offered it, without naming the discoverer, in a small shop in Soho in April 1803. After harsh criticism from Richard Chenevix that palladium is an alloy of platinum and mercury, Wollaston anonymously offered a reward of 20 British pounds for 20 grains of synthetic palladium alloy. Chenevix received the Copley Medal in 1803 after he published his experiments on palladium. Wollaston published the discovery of rhodium in 1804 and mentions some of his work on palladium. He disclosed that he was the discoverer of palladium in a publication in 1805.
Palladium chloride was at one time prescribed as a tuberculosis treatment at the rate of 0.065 g per day (approximately one milligram per kilogram of body weight). This treatment had many negative side-effects, and was later replaced by more effective drugs.
Most palladium is used for catalytic converters in the automobile industry. In the run up to year 2000, the Russian supply of palladium to the global market was repeatedly delayed and disrupted because for political reasons, the export quota was not granted on time. The ensuing market panic drove the price to an all-time high of $1100 per troy ounce in January 2001. Around that time, the Ford Motor Company, fearing that automobile production would be disrupted by a palladium shortage, stockpiled the metal. When prices fell in early 2001, Ford lost nearly US$1 billion.
World demand for palladium increased from 100 tons in 1990 to nearly 300 tons in 2000. The global production of palladium from mines was 222 tonnes in 2006 according to theUnited States Geological Survey. Many are concerned about a steady supply of palladium in the wake of Russia's military maneuvers in Ukraine, partly as sanctions could hamper Russian palladium exports; any restrictions on Russian palladium exports would exacerbate what is already expected to be a large palladium deficit in 2014.[needs update]
With a 44% world share of palladium in 2007, Russia was the top producer, followed by South Africa with 40%. Canada with 6% and the U.S. with 5% are the other substantial producers of palladium.
When dispersed on conductive materials, palladium is an excellent electrocatalyst for oxidation of primary alcohols in alkaline media. In 2010, palladium-catalysed organic reactions were recognised by the Nobel Prize in Chemistry. Palladium is also a versatile metal forhomogeneous catalysis, used in combination with a broad variety of ligands for highly selective chemical transformations. A 2008 study showed that palladium is an effective catalyst for carbon-fluoride bonds.
The second greatest application of palladium in electronics is in multilayer ceramic capacitors in which palladium (and palladium-silver alloy) is used for electrodes. Palladium (sometimes alloyed with nickel) is used for component and connector plating in consumer electronics and in soldering materials. The electronic sector consumed 1.07 million troy ounces (33.2 tonnes) of palladium in 2006, according to aJohnson Matthey report.
Palladium readily absorbs hydrogen at room temperatures, forming palladium hydride PdHx with x less than 1. While this property is common to many transition metals, palladium has a uniquely high absorption capacity and does not lose its ductility until x approaches 1. This property has been investigated in designing an efficient, inexpensive, and safe hydrogen fuel storage medium, though palladium itself is currently prohibitively expensive for this purpose. The content of hydrogen in palladium can be linked to magnetic susceptibility, which decreases with the increase of hydrogen and becomes zero for PdH0.62. At any higher ratio, the solid solution becomes diamagnetic.
Palladium has been used as a precious metal in jewelry since 1939 as an alternative to platinum in the alloys called "white gold", where the naturally white color of palladium does not require rhodium plating. Palladium is much less dense than platinum. Similar to gold, palladium can be beaten into leaf as thin as 100 nm (1⁄250,000 in). Unlike platinum, palladium may discolor at temperatures above 400 °C (752 °F); it is relatively brittle.[clarification needed]
Palladium is one of the three most popular alloying metals in white gold (nickel and silver can also be used). Palladium-gold is more expensive than nickel-gold, but seldom causes allergic reactions (though certain cross-allergies with nickel may occur).
When platinum was declared a strategic government resource during World War II, many jewelry bands were made out of palladium. As recently as September 2001, palladium was more expensive than platinum and rarely used in jewelry because of the technical difficulty of casting. Currently, the casting problem has been resolved and use in jewelry has increased because platinum has increased in price while palladium decreased.
Prior to 2004, the principal use of palladium in jewelry was the manufacture of white gold. In early 2004, when gold and platinum prices rose steeply, China began fabricating volumes of palladium jewelry, consuming 37 tonnes in 2005. Changes in the relative price of platinum after 2008 lowered demand for palladium to 17.4 tonnes in 2009.
In January 2010, hallmarks for palladium were introduced by assay offices in the United Kingdom, and hallmarking became mandatory for all jewelry advertising pure or alloyed palladium. Articles can be marked as 500, 950, or 999 parts of palladium per thousand of the alloy.
Fountain pennibs made from gold are sometimes plated with palladium when a silver (rather than gold) appearance is desired. Sheaffer has used palladium plating for decades, either as an accent on otherwise gold nibs or covering the gold completely.
Palladium is a metal with low toxicity. It is poorly absorbed by human body when digested. Plants such as the water hyacinth are killed by low levels of palladium salts, but most other plants tolerate it, although tests show that, at levels above 0.0003%, growth is affected. High doses of palladium could be poisonous; tests on rodents suggest it may becarcinogenic, though no clear evidence indicates the element harms humans.
Like other platinum-group metal, bulk Pd is quite inert. Although contact dermatitis has been reported, data on the effects are limited. It has been shown that people with an allergic reaction to palladium also react to nickel, making it advisable to avoid the use of dental alloys containing palladium on those so allergic.
Some palladium is emitted with the exhaust gases of cars with catalytic converters. Between 4 and 108 ng/km of palladium particulate is released by such cars, while the total uptake from food is estimated to be less than 2 µg per person a day. The second possible source of palladium is dental restoration, from which the uptake of palladium is estimated to be less than 15 µg per person per day. People working with palladium or its compounds might have a considerably greater uptake. For soluble compounds such aspalladium chloride, 99% is eliminated from the body within 3 days.