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.


Molybdenum, symbol Mo, metallic element with chemical properties similar to those of chromium. Molybdenum is one of the transition elements of the periodic table (see Periodic Law). The atomic number of molybdenum is 42.

Molybdenum was discovered in 1778 by the Swedish chemist Carl Wilhelm Scheele. It is a silvery white, tough, malleable metal. Molybdenum is dissolved by dilute nitric acid and aqua regia, and is attacked by fused alkalies; it is not attacked by air at ordinary temperatures, but at elevated temperatures it oxidizes to form molybdenum oxide. Molybdenum melts at about 2610° C (about 4730° F), boils at about 4640° C (about 8380° F), and has a specific gravity, or relative density, of 10.2.

Molybdenum does not occur free in nature, but in the form of its ores, the most important of which are molybdenite and wulfenite. It ranks 56th in order of abundance of the elements in the crust of the earth and is an important trace element in soils, where it contributes to the growth of plants.

The metal is used chiefly in alloying steel. The alloy withstands high temperatures and pressures and is very strong, making it useful for structural work, aircraft parts, and forged automobile parts. Molybdenum wire is used in electron tubes, and the metal also serves as electrodes in glass furnaces. Molybdenum sulfide is used as a lubricant in environments requiring high temperatures.