|Name, Symbol, Number||palladium, Pd, 46|
|Chemical series||transition metals|
|Group, Period, Block||10, 5, d|
|Appearance||silvery white metallic
|Atomic mass||106.42(1) g/mol|
|Electron configuration||[Kr] 4d10|
|Electrons per shell||2, 8, 18, 18, 0|
|Density (near r.t.)||12.023 g/cm³|
|Liquid density at m.p.||10.38 g/cm³|
|Melting point||1828.05 K
(1554.9 °C, 2830.82 °F)
|Boiling point||3236 K
(2963 °C, 5365 °F)
|Heat of fusion||16.74 kJ/mol|
|Heat of vaporization||362 kJ/mol|
|Heat capacity||(25 °C) 25.98 J/(mol·K)|
|Crystal structure||cubic face centered|
(mildly basic oxide)
|Electronegativity||2.20 (Pauling scale)|
|Ionization energies||1st: 804.4 kJ/mol|
|2nd: 1870 kJ/mol|
|3rd: 3177 kJ/mol|
|Atomic radius||140 pm|
|Atomic radius (calc.)||169 pm|
|Covalent radius||131 pm|
|Van der Waals radius||163 pm|
|Magnetic ordering||no data|
|Electrical resistivity||(20 °C) 105.4 nΩ·m|
|Thermal conductivity||(300 K) 71.8 W/(m·K)|
|Thermal expansion||(25 °C) 11.8 µm/(m·K)|
|Speed of sound (thin rod)||(20 °C) 3070 m/s|
|Speed of sound (thin rod)||(r.t.) 121 m/s|
|Shear modulus||44 GPa|
|Bulk modulus||180 GPa|
|Vickers hardness||461 MPa|
|Brinell hardness||37.3 MPa|
|CAS registry number||7440-05-3|
Palladium (chemical symbol Pd, atomic number 46) is a rare, silver-white metal. It is a member of the platinum group of elements and resembles platinum chemically. It is extracted from some copper and nickel ores. It has the unusual ability to absorb large quantities of hydrogen gas, expanding visibly as it does so.
Palladium and its compounds are extremely valuable catalysts for various chemical reactions, and palladium can be found in automobile catalytic converters. Palladium alloys are used in jewelry. In addition, this element is useful in a number of other applications, including dentistry, watchmaking, aircraft spark plugs, surgical instruments, and electrical contacts. Hydrogen absorbed in palladium is highly reactive and is used in reduction reactions. Palladium dichloride may be used in detectors for carbon monoxide and tests for the corrosion-resistance of stainless steel.
Palladium occurs in nature as a free metal and alloyed with gold, platinum, and other platinum group metals. It has been found in placer deposits in the Ural Mountains of western Russia, and in some parts of Australia, Ethiopia, and South and North America. In addition, it is commercially produced from nickel-copper deposits in South Africa, Ontario, and Siberia. Although the proportion of palladium in the nickel-copper ores is low, the processing of large volumes of ore makes this extraction profitable.
Palladium was discovered by William Hyde Wollaston in 1803 in England. Using a platinum ore that presumably came from South America, he performed a series of chemical reactions and obtained the compound palladium cyanide. Finally, by heating palladium cyanide, he was able to isolate palladium metal. He named the element in 1804, deriving the word from Pallas, the name of an asteroid discovered two years earlier.
Palladium is classified as a transition metal. In the periodic table, it lies in period five between rhodium and silver and is closely related to the latter two elements. In addition, it is situated in group ten (former group 8B), between nickel and platinum.
This element resembles platinum, but among the platinum group metals, it has the lowest density and melting point. It is soft and ductile when annealed, but it greatly increases in strength and hardness when cold-worked. Palladium is chemically attacked by sulfuric acid, nitric acid, and hydrochloric acid in which it dissolves slowly. When it is heated to 800°C, a layer of palladium(II) oxide (PdO) is produced.
Palladium has the uncommon ability to absorb up to 900 times its own volume of hydrogen at room temperature. As it absorbs hydrogen, it expands visibly, like a sponge that swells when soaking up water. In so doing, it is thought to form palladium hydride (PdH2), but scientists are unsure if this is a true chemical compound.
Common oxidation states of palladium are 0, +1, +2, and +4. Although it was once thought that +3 was one of the fundamental oxidation states of palladium, there is no evidence for that. When several palladium compounds were investigated by the technique of X-ray diffraction, a dimer of palladium(II) and palladium(IV) was discovered instead. Recently, researchers synthesized compounds in which palladium has an oxidation state of +6.
Naturally occurring palladium is composed of six stable isotopes: 102Pd, 104Pd, 105Pd, 106Pd, 108Pd, and 110Pd. In addition, numerous radioactive isotopes are known, with mass numbers ranging from 91 to 124. The longest-lived 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. Most of the other radioisotopes have half-lives that are less than a half hour.
In March 1989, researchers Stanley Pons and Martin Fleischmann announced that they had found a way to carry out a safe nuclear reaction at low temperatures in a tabletop experiment. The reaction, which was thought to involve the fusion of hydrogen nuclei, was dubbed "cold fusion." Palladium electrodes played an important role in this experiment. It was postulated that hydrogen atoms could be "squeezed" between the palladium atoms to help them fuse at lower temperatures than usually required for fusion to proceed. Since then, many other experiments have been conducted to test the possibility of cold fusion, but scientists remain divided on the issue of whether the observations are based on genuine cases of nuclear fusion.
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