Molybdenum metal properties
Molybdenum (Mo) is a chemical element or strong and silvery transition metal of Group 6 (VIB) of the periodic table, used for making stainless steel and alloy steels for high-speed tools. The name molybdenum is given from the Greek word molybdos meaning lead. It is the element in the second transition series in which all the d-electrons participate in metallic bonding.
The atomic number of molybdenum is 42 and the valence shell electronic configuration [Kr] 4d5 5s1. The metal form typically metallic body-centered cubic crystal lattice. It is hard and relatively unreactive due to the protective surface film of oxide. The most common oxidation number or state of molybdenum metal is +6 (VI). The oxidation states V and IV are also stable. Strong reducing properties of molybdenum metal not met before III and II states.
|Properties of Molybdenum|
|Electronic configuration||[Kr] 4d5 5s1|
|Melting point||2896 K (2623 °C, 4753 °F)|
|Boiling point||4912 K (4639 °C, 8382 °F)|
|Molar heat capacity||24.06 J mol-1K-1|
|Electrical resistivity||53.40 nΩ·m|
|Atomic radius||139 pm|
|Covalent radius||154±5 pm|
|Electronegativity||Pauling scale – 2.16|
|Ionization energy||1st – 684.3 kJ/mol
2nd – 1560 kJ/mol
3rd – 2618 kJ/mol
History and Occurrence
The soft black molybdenite (MoS2) is the principal ore of molybdenum used for writing in the old time. It was similar to that of lead ore (galena or PbS) and graphite. The mineral was first studied in 1754 by Swedish mineralogist A. Cronstedt. Following him, in 1778 Swedish chemist Carl Wilhelm Scheele observed that it was neither galena nor graphite. He prepared the oxide of molybdenum metal. Pure molybdenum was prepared by Berzelius in 1817 by reducing the oxide with hydrogen. The name of the metal is given from the Greek word molybdos meaning lead.
The metal molybdenum and tungstentungsten are rare in the earth’s atmosphere. It is the 54th most abundant element in the Earth’s crust with an occurrence of 1.5 parts per million. The most important ore of Mo is molybdenite (MoS2). It is found mostly in Colorado (USA), Canada and Chile. Other minerals of the metal are wulfenite (PbMoO4) and molybdite (MoO3).
Molybdenum has 35 known isotopes with an atomic mass of 83 to 113. It has seven naturally occurring isotopes with the atomic mass 92, 94, 95, 96, 97, 98, and 100. The radioactive isotopes of the metal are obtained by different types of nuclear reactions. 93Mo is the most stable radioisotopes of the metal with a half-life of 4,000 years.
Production of Molybdenum
Molybdenite was concentrated by foam floatation and roasted to MoO3. First, it is reduced to MoO2 by heating with hydrogen at 500 °C. In the second step, MoO2 is reduced by hydrogen at 1100 °C to get the metal. Pure molybdenum metal obtained by hydrogen reduction of ammonium molybdate. Carbon reduction of MoO3 gives mostly carbides.
Ferromolybdenum is prepared by reducing molybdenite with iron and coke in an electric furnace. It is also prepared by reducing a mixture of molybdenum and iron oxides with aluminum (thermite reaction). The metal has a very high melting point and is obtained initially in the form of molybdenum powder. It is converted to massive states by compression under hydrogen at high temperatures.
Chemistry and Chemical Compounds
The chemistry of transition metals molybdenum and tungsten are very similar. The metals are slightly attacked by weak acids and bases. Both of them dissolved in nitric acid – hydrogen fluoride mixture and in fused alkalies. The oxide compounds are acidic but the salts derived from acids are complex polyanions.
The common and stable oxidation states of these metals are +6 (VI). The oxidation states V and IV are also stable. Strong reducing properties of molybdenum are not observed before III and II states. In VI state, molybdenum metal forms oxides, oxoacids, hexafluorides, and hexachlorides.
Oxides and Oxoacids
Molybdenum forms hexavalent oxide like MoO3. It is precipitated from alkali molybdates by the addition of acids. The precipitated H2MoO4 may be ignited to form oxides. The oxide may also be prepared by ignition of metal in air or oxygen. All the oxide are acidic and dissolved in alkali to give molybdates. The oxide CrO3 is highly oxidizing in nature. The oxides like MoO2 and WO2 have no oxidizing properties. The lower oxide MoO2 was obtained by the reduction of trioxide with hydrogen. The acidic nature of the oxide decreases with the decreasing oxidation state.
With the high oxidation number, molybdenum halides are expectedly covalent in nature. Therefore, MoF6 is low boiling, nonelectrolyte, diamagnetic, and extremely dissolved to hydrolysis. MoCl5 is monomeric in vapor state but a bridged dimer in the crystalline state. The lower halides are very complicated structures.
The higher halides are obtained by the direct halogenation of the metal. The lower halides are obtained by the reduction of higher halides with metal or hydrogen under optimized temperature and pressure.
It form monomeric MO4-2 anions in an alkaline medium. On acidification, the anions polymerize to give polyacids. These give rise to a complex like dioxomolybdenum(VI). The pentavalent complexes are mostly of the oxo-molybdenum or di-μ-oxodimolybdenum type. Some common examples of molybdenum complexes in the +4 state are [MoCl5O]-2, [Mo(acac)2ClO]-2, etc. Tetravalent complexes are both oxo and nonoxo types.
In a lower oxidation state, molybdenum forms Mo6X12 cluster compounds (where X = chlorine, bromine, and iodine). In this type of compound six Mo atom forming cluster type compound like (MoX8)+4X4–. The cyano complex of molybdenum may be prepared by reducing the Mo(IV) complex. It has a pentagonal bipyramid structure.
Common uses of molybdenum metal
- The main consumption of molybdenum used in the manufacturing of stainless steel and other types of steel used in high-speed tools. The oxide MoO3 and ferromolybdenum are directly used for this purpose.
- Molybdenum increases the mechanical strength, electrical conductance, and corrosion resistance of steels. Therefore, Ni-Mo steel is used for making gun-barrels and propeller shafts. Cr-Mo steel is used for making aircraft.
- Pure molybdenum metal is used to treat crude petroleum with hydrogen gas. The metal and hydrogen convert sulfur-containing organic compounds to H2S.
- Molybdenum metal is used as a chemical catalyst. The compound MoS2 has a layer of the crystal lattice and is used for making high-temperature lubricants.
Biological function of molybdenum
It is the only element or metal in the second transition series which has natural biological functions. More than thirty microorganisms, plants, and animals are known to contain the metal. Nitrogen fixation enzyme nitrogenases containing a characteristic polymetallic cluster with the iron-molybdenum cofactor. Other enzymes function like oxidase, reductase, dehydrogenase has variant types of molybdenum metal cofactors.