Palladium Technical Information
Palladium is a chemical element with symbol Pd and the atomic number 46. It is a rare silver-white transition metal of the platinum group, resembling platinum chemically. It was discovered in platinum ores in 1803, and named after the asteroid Pallas by William Hyde Wollaston.
Palladium is usually found as a free metal, alloyed with others in the platinum group. It is commercially extracted from copper-nickel ores. Palladium has a great affinity for hydrogen, being able to absorb 900 times its own volume of the gas. Palladium metal and its complexes are often used in catalysis such as in catalytic converters on cars, palladium on carbon used in organic chemistry, and other coupling reactions. As a precious metal, it is sometimes used in jewelry, and has the ISO currency code of XPD.
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 degrees Celcius 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 synthesised.
Palladiums Technical Characteristics are as follows:
Chemical series: transition metals
Block: d 10, 5, d
Appearance: silvery white metallic
Standard atomic weight: 106.42(1) ?g mol-1
Electron configuration: [Kr] 4d10
Electrons per shell: 2, 8, 18, 18, 0
Density (near r.t.): 12.023 g cm-3
Liquid density at m.p.: 10.38 g cm-3
Melting point: 1828.05K (1554.9 C, 2830.82 F)
Boiling point: 3236K (2963 C, 5365 F)
Heat of fusion: 16.74 kJ mol-1
Heat of vaporization: 362 kJ mol-1
Heat capacity: (25? C) 25.98 J mol-1 K-1
P(Pa): 1 10 100 1 k 10 k 100 k
at T(K): 1721 1897 2117 2395 2753 3234
Crystal structure: cubic face centered
Oxidation states: 2, 4 (mildly basic oxide)
Electronegativity: 2.20 (Pauling scale)
1st: 804.4 kJ/mol
Atomic radius: 140 pm
2nd: 1870 kJ/mol
3rd: 3177 kJ/mol
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 n?O m
Thermal conductivity: (300?K) 71.8 W m-1 K-1
Thermal expansion: (25? C) 11.8 um m-1 K-1
Speed of sound (thin rod): (20 C) 3070 m/s
Young's modulus: 121 GPa
Shear modulus: 44 GPa
Bulk modulus: 180 GPa
Poisson ratio: 0.39
Mohs hardness: 4.75
Vickers hardness: 461 MPa
Brinell hardness: 37.3 MPa
CAS registry number: 7440-05-3
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