|Name, Symbol, Number||Palladium, Pd, 46|
|Chemical series||Transition metals|
|Group, Period, Block||10, 5 , d|
|Density, Hardness||12023 kg/m3, 4.75|
|Appearance||Silvery white metallic|
|Atomic weight||106.42 amu|
|Atomic radius (calc.)||140 (169) pm|
|Covalent radius||131 pm|
|van der Waals radius||163 pm|
|e- 's per energy level||2, 8, 18, 18, 0|
|Oxidation states (Oxide)||±1 (mildly basic)|
|Crystal structure||Cubic face centered|
|State of matter||Solid (__)|
|Melting point||1828.05 K (2830.82 °F)|
|Boiling point||3236 K (5365 °F)|
|Molar volume||8.56 ×1010-3 m3/mol|
|Heat of vaporization||357 kJ/mol|
|Heat of fusion||17.6 kJ/mol|
|Vapor pressure||1.33 Pa at 1825 K|
|Speed of sound||3070 m/s at 293.15 K|
|Electronegativity||2.20 (Pauling scale)|
|Specific heat capacity||244 J/(kg*K)|
|Electrical conductivity||9.5 106/m ohm|
|Thermal conductivity||71.8 W/(m*K)|
|1st ionization potential||804.4 kJ/mol|
|2nd ionization potential||1870 kJ/mol|
|3rd ionization potential||3177 kJ/mol|
|Most Stable Isotopes|
|SI units & STP are used except where noted.|
- For alternate meanings see: Palladium (disambiguation)
|Table of contents|
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Palladium is a soft steel-white metal that resembles platinum, doesn't tarnish in air, and 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 and nitric acid but dissolves slowly in hydrochloric acid. This metal also does not react with oxygen at normal temperatures.
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 - Pd2H - but it is not yet clear if this is a true chemical compound.
Common oxidation states of palladium are +2, +3 and +4. Recently, palladium compounds in which palladium has oxidation state +6 were synthesized.
When it is finely divided, palladium forms a good catalyst and is used to speed up hydrogenation and dehydrogenation reactions, as well as in petroleum cracking. It is also alloyed and used in jewelry. Other uses;
- White gold is an alloy of gold that is decolorized by the addition of palladium.
- Similar to gold, palladium can be beaten into a thin leaf form as thin as 1/250,000 in.
- Hydrogen easily diffuses through heated palladium; thus, it provides a means of purifying the gas.
- Telecommunications switching-system equipment uses palladium.
- Palladium is also used in dentistry, watchmaking, in aircraft sparkplugs and in the production of surgical instruments and electrical contacts.
HistoryPalladium 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 element 46 in crude platinum ore from South America. He did this by dissolving the ore in aqua regia, neutralizing the solution with sodium hydroxide, NaOH, precipitating platinum as ammonium chloroplatinate through treatment with ammonium chloride, NH4Cl, and then adding mercuric cyanide to form the compound palladium cyanide. Finally, he heated the resulting compound in order to extract palladium metal.
The compound palladium chloride was at one time prescribed as a tuberculosis treatment at the rate of 0.065 g per day (approximately 1 mg per kg of body weight). This treatment did not have too many ill side effects but was later replaced by more effected drugs.
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. However it is commercially produced from nickel-copper deposits found in South Africa and Ontario (the huge volume of ore processed makes this extraction profitable in spite of its low concention in these ores).
Naturally-occurring palladium is composed of six isotopes. The most stable radioisotopes are Pd-107 with a half-life of 6.5 million years, Pd-103 with a half-life of 17 days, and Pd-100 with a half-life of 3.63 days. Eighteen other radioisotopes have been characterized with atomic weights ranging from 92.936 amu (Pd-93) to 119.924 amu (Pd-120). Most of these have half-lifes that are less than a half an hour except Pd-101 (half-life: 8.47 hours), Pd-109 (half-life: 13.7 hours), and Pd-112 (half-life: 21 hours).
The primary decay mode before the most abundant stable isotope, Pd-106, is electron capture and the primary mode after is beta decay. The primary decay product before Pd-106 is rhodium and the primary product after is silver.
Radiogenic Ag-107 is a decay product of Pd-107 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. Pd-107 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.