Palladium
. Palladium (element), symbol Pd, relatively rare, silvery white, soft metallic element. The atomic number of palladium is 46. The element is one of the transition elements of the periodic table (see Periodic Law ). Palladium was discovered in 1804 by the British chemist William Hyde Wollaston . Palladium has a hardness of 4.8. Like platinum , it is ductile, malleable, and resistant to corrosion; it fuses more easily than platinum and can be welded easily. Finely divided palladium is an excellent adsorbent for some gases; it adsorbs 1000 to 3000 times its volume of hydrogen or acetylene gas when heated to 100° C (212° F). Palladium is dissolved readily by aqua regia . It forms divalent and tetravalent compounds and resembles platinum chemically. Palladium ranks about 71st in natural abundance among the elements in crustal rock. Palladium melts at about 1554° C (about 2829° F), boils at about 2970° C (about 5378° F), and has a specific gravity of 12.02. The atomic weight of Palladium is