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.


Platinum, symbol Pt, relatively rare, chemically inert metallic element that is more valuable than gold. The atomic number of platinum is 78. The element is one of the transition elements in group 10 (or VIIIb) of the periodic table (see Periodic Law).

Platinum is the most important of the group of elements called the platinum metals, the other members of which are ruthenium, rhodium, palladium, osmium, and iridium. Platinum metals were probably used in alloyed forms in ancient Greece and Rome and were first mentioned in European literature in the early 16th century. The separation of the other platinum metals from platinum and from each other was accomplished in the early 19th century.


Platinum is a grayish-white metal with a hardness of 4.3. It has a high fusing point, is malleable and ductile, expands slightly upon heating, and has high electrical resistance. Chemically the metal is relatively inert and resists attack by air, water, single acids, and ordinary reagents. It dissolves slowly in aqua regia, forming chloroplatinic acid; is attacked by halogens; and combines upon ignition with sodium hydroxide, sodium nitrate, or sodium cyanide. Platinum melts at about 1772° C (about 3222° F), boils at about 3827° C (about 6921° F), and has a specific gravity of about 21.45. The atomic weight of platinum is 195.09.

Platinum ranks about 72nd in natural abundance among the elements in crustal rock. Except for the mineral sperrylite, which is platinum arsenide and is found only sparingly in a few localities, platinum occurs in the metallic state, often alloyed with other platinum metals. Nuggets of the metal weighing up to 9.5 kg (21 lb) have been found.


Because of its chemical inertness and high fusing point, platinum is valuable for laboratory apparatus, such as crucibles, tongs, funnels, combustion boats, and evaporating dishes. Small amounts of iridium are usually added to increase its hardness and durability. Platinum is also used for contact points in electrical apparatus and in instruments used for measuring high temperatures. Finely divided platinum in the form of platinum sponge or platinum black is used extensively as a catalyst in the chemical industry. A considerable amount of the platinum used in the United States goes into jewelry, in which it is often alloyed with gold. It is also used for dental fillings.