Molybdenum (Chemical symbol: Mo)
In ancient times, molybdenum ore was indistinguishable from materials such as lead, galena (a lead ore) and graphite. Collectively, these substances were known by the Greek word "molybdos," which means “lead-like.” Molybdenum was first identified by Swedish chemist Karl Wilhelm Scheele in 1778 when he was able to isolate the oxide from graphite and lead. Later, his friend Peter Jacob Hjelm was able to isolate an impure extract of the metal by reducing the oxide with carbon. The newly discovered element was announced in the autumn of 1781. Molybdenum is a hard, silvery-white metallic element (Fig. 1) that is classified as a "transition metal." Transition metals are ductile, malleable and able to conduct electricity and heat.
Molybdenum has one of the highest melting points of all pure elements, 2623°C (4753°F). Molybdenum is available in many forms, including foil, sheet, wire, mesh, rod, powder and tubes. Over two-thirds of all molybdenum is used as an alloying addition in high-strength alloys and high-temperature steels. Pure molybdenum (Fig. 2) is widely used in the construction of furnace tooling and parts and as a feedstock for the fabrication of parts for the electronics and semiconductor industries.
Molybdenum is sold as a gray powder and used as an alloying addition to steel; catalysts and electrodes; and for aircraft, missile and nuclear components. Other uses include filaments and electronic applications. Molybdenum pigments, ranging from reddish-yellow to a bright red-orange, are used in paints, inks, plastics and rubber compounds. Molybdenum disulfide (MoS2) is a good lubricant, especially at high temperatures. It is also a trace element in plant and animal metabolism and is found in legumes (such as beans, lentils and peas) as well as grain products and nuts.
Molybdenum does not occur free in nature but in the form of its ores, the most important of which are molybdenite (MoS2), wulfenite (MoO4Pb) and powellite (CaMnO4). It ranks 56th in order of abundance of the elements in the crust of the earth. Molybdenum is mined directly and is also recovered as a byproduct of copper mining. The percentage of molybdenum present in its ores varies from 0.01% to about 0.5%. About half of the world's molybdenum is mined in the U.S.
Here are a few important facts about molybdenum.
- Atomic number: 42
- Density (g/cc): 10.22
- Atomic weight (u): 95.94
- Melting point (K): 2890.15
- Boiling point (K): 4885.15
- Bonding (covalent) radius (A): 1 .30
- Atomic radius (A): 2.01
- Heat of vaporization (kJ/mol): 598.0
- Ionization potential (V): 7.099
- Heat of fusion (kJ/mol): 32.0
- Electronegativity: +2.16
- Specific heat (J/gK) 0 .25
- Oxidation states +6, +5, +4, +3, +2
- Electron configuration. : [Kr] 4d55s1
Other noteworthy facts include:
- The crystal structure of molybdenum is body-centered cubic.
- Molybdenum is commonly called “moly” and has the lowest thermal-expansion coefficient of the engineering metals.
- Molybdenum has 42 protons, 42 electrons and 54 neutrons.
- Herring, Daniel, Vacuum Heat Treatment Volume II, BNP Media, 2016
- (courtesy images-of-elements.com)
- Stanford Materials Corporation (www.standfordmaterials.com)