Molybdenum, element number 42 of the periodic table, lies in the table's second transition series, in Group 6A between chromium and tungsten.
It has one of the highest melting temperatures of all the elements, yet unlike most other high-melting point metals, its density is only 25% greater than iron's. Its coefficient of thermal expansion is the lowest of the engineering materials, while its thermal conductivity exceeds all but a handful of elements.
Molybdenum Properties
|
Atomic number |
42 |
Atomic weight |
95.96 |
Crystal structure |
Body-centered cubic (BCC) |
Lattice constant |
a = 3.1470 Å |
Density: |
10.22 g/cm3 |
Melting temperature |
2623 °C |
Coefficient of thermal expansion |
4.8 x 10-6 / K at 25 °C |
Thermal conductivity |
138 W/m K at 20°C |
Physical properties
As a solid, molybdenum has a silvery-white metallic appearance. It more commonly occurs as a dark gray or black powder with a metallic luster. Its melting point is about 2,610°C (about 4,700°F) and the boiling point is 4,800 to 5,560°C (8,600 to 10,000°F). Its density is 10.28 grams per cubic centimeter.
Chemical properties
Molybdenum does not dissolve in most common chemical reagents. A chemical reagent is a substance used to study other substances, such as an acid or an alkali. For example, molybdenum does not dissolve in hydrochloric acid, hydrofluoric acid, ammonia, sodium hydroxide, or dilute sulfuric acid. These chemicals are reagents often used to test how reactive a substance is. Molybdenum does dissolve in hot strong sulfuric or nitric acids, however. The metal does not react with oxygen at room temperatures, but does react with oxygen at high temperatures.
Uses
About 75 percent of the molybdenum used in the United States in 1996 was made into alloys of steel and iron. Nearly half of these alloys, in turn, were used to make stainless and heat-resistant steel. A typical use is in airplane, spacecraft, and missile parts. Another important use of molybdenum alloys is in the production of specialized tools. Spark plugs, propeller shafts, rifle barrels, electrical equipment used at high temperatures, and boiler plates are all made of molybdenum steel.
Another important use of molybdenum is in catalysts. A catalyst is a substance used to speed up or slow down a chemical reaction. The catalyst does not undergo any change itself during the reaction. Molybdenum catalysts are used in a wide range of chemical operations, in the petroleum industry, and in the production of polymers and plastics.
When added to steel and cast irons, molybdenum enhances strength, hardenability, weldability, toughness, elevated temperature strength, and corrosion resistance. In nickel-base alloys, it improves resistance to both corrosion and high-temperature creep deformation.
Molybdenum-based alloys have a unique combination of properties, including high strength at elevated temperatures, high thermal and electrical conductivity, and low thermal expansion. Molybdenum metal and its alloys are the first choice in many demanding specialized applications.
Chemically, the outstanding feature of molybdenum is its extraordinary versatility:
- Oxidation states from –II to VI
- Coordination numbers from 4 to 8
- Varied stereochemistry
- The ability to form compounds with inorganic and organic ligands, with particular preference for oxygen, sulfur, fluorine and chlorine donor atoms
- Formation of bi- and poly-nuclear compounds containing bridging oxide or chloride ligands and/or molybdenum-molybdenum bonds.
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