Characteristics of Molybdenum
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- Category: Molybdenum knowledge
- Published on 27 August 2014
- Written by jwy
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Physical properties
In its pure form, molybdenum is a silvery-grey metal with a Mohs hardness of 5.5. It has a melting point of 2,623 °C (4,753 °F);Of the naturally occurring elements, only tantalum, osmium, rhenium, tungsten, and carbon have higher melting points. Weak oxidation of molybdenum starts at 300 °C (572 °F). It has one of the lowest coefficients of thermal expansion among commercially used metals. The tensile strength of molybdenum wires increases about 3 times, from about 10 to 30 GPa, when their diameter decreases from 50–100 nm to 10 nm.
Isotopes
There are 35 known isotopes of molybdenum, ranging in atomic mass from 83 to 117, as well as four metastable nuclear isomers. Seven isotopes occur naturally, with atomic masses of 92, 94, 95, 96, 97, 98, and 100. Of these naturally occurring isotopes, only molybdenum-100 is unstable.
Molybdenum-98 is the most abundant isotope, comprising 24.14% of all molybdenum. Molybdenum-100 has a half-life of about 1019 y and undergoes double beta decay into ruthenium-100. Molybdenum isotopes with mass numbers from 111 to 117 all have half-lives of approximately 150 ns. All unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium.
As also noted below, the most common isotopic molybdenum application involves molybdenum-99, which is a fission product. It is a parent radioisotope to the short-lived gamma-emitting daughter radioisotope technetium-99m, a nuclear isomer used in various imaging applications in medicine. In 2008, the Delft University of Technology applied for a patent on the molybdenum-98-based production of molybdenum-99.
Molybdenum is a transition metal with an electronegativity of 2.16 on the Pauling scale and a standard atomic weight of 95.95 g/mol. It does not visibly react with oxygen or water at room temperature, and the bulk oxidation occurs at temperatures above 600 °C, resulting in molybdenum trioxide:
2 Mo + 3 O2 → 2 MoO3
The trioxide is volatile and sublimates at high temperatures. This prevents formation of a continuous protective oxide layer, which would stop the bulk oxidation of metal. Molybdenum has several oxidation states, the most stable being +4 and +6 (bolded in the table). The chemistry and the compounds show more similarity to those of tungsten than that of chromium. An example is the instability of molybdenum(III) and tungsten(III) compounds as compared with the stability of the chromium(III) compounds. The highest oxidation state is common in the molybdenum(VI) oxide (MoO3), whereas the normal sulfur compound is molybdenum disulfide MoS2.
Molybdenum(VI) oxide is soluble in strong alkaline water, forming molybdates (MoO42). Molybdates are weaker oxidants than chromates, but they show a similar tendency to form complex oxyanions by condensation at lower pH values, such as [Mo7O24]6 and [Mo8O26]4. Polymolybdates can incorporate other ions into their structure, forming polyoxometalates. The dark-blue phosphorus-containing heteropolymolybdate P[Mo12O40]3 is used for the spectroscopic detection of phosphorus. The broad range of oxidation states of molybdenum is reflected in various molybdenum chlorides:
Molybdenum(II) chloride MoCl2 (yellow solid)
Molybdenum(III) chloride MoCl3 (dark red solid)
Molybdenum(IV) chloride MoCl4 (black solid)
Molybdenum(V) chloride MoCl5 (dark green solid)
Molybdenum(VI) chloride MoCl6 (brown solid)
The structure of the MoCl2 is composed of Mo6Cl84+ clusters with four chloride ions to compensate the charge.
Like chromium and some other transition metals, molybdenum is able to form quadruple bonds, such as in Mo2(CH3COO)4. This compound can be transformed into Mo2Cl84, which also has a quadruple bond.
The oxidation state 0 is possible with carbon monoxide as ligand, such as in molybdenum hexacarbonyl, Mo(CO)6.
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