Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


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 106.4. The metal occurs in the pure state in platinum ores and in the combined state in Canadian nickel ore.

The chief use of the metal is as a catalyst in automobiles to reduce pollution. It is also used in the field of communications, where it is used to face electrical contacts in automatic switchgear; in dentistry; for nonmagnetic springs in clocks and watches; for coating special mirrors; and in jewelry, alloyed with gold, in what is called white gold.