Rhenium, a very rare silver-white metallic element. The density and melting point of rhenium are among the highest of all the chemical elements. Rhenium does not occur free in nature. Its primary source is molybdenite. Rhenium is used as a catalyst for producing high-octane gasoline. It is also used in both molybdenum and tungsten alloys for such items as electrical contacts and high-temperature thermocouples.

Rhenium was discovered in 1925 by three German scientists: Walter Noddack, Ida Tacke, and Otto Berg.

Symbol: Re. Atomic number: 75. Atomic weight: 186.207. Specific gravity: 21.02. Melting point: 5,756 F. (3,180 C.). Boiling point: about 10,170 F. (5,630 C.). Rhenium has two naturally occurring isotopes, Re-185 and Re-187. Rhenium belongs to Group VIIB of the Periodic Table and may have a valence of 1, 2, 3, 4, 5, 6, 7, or -1.