128 | The History and Use of Our Earth’s Chemical Elements Properties
128 | The History and Use of Our Earth’s Chemical Elements Properties
Molybdenum is in the middle of the triad elements of group 6. These three metals (from periods 4, 5, and 6) are chromium, molybdenum, and tungsten, which, in their pure states, are relatively hard, but not as hard as iron. They are silvery-white as pure metals, and they have similar oxidation states. Their electronegativity is also similar—Cr = 1.6, Mo = 1.8, and W = 1.7—which is related to their reactivity with nonmetals.
Molybdenum is malleable and ductile, but because of its relatively high melting point, it is usually formed into shapes by using powder metallurgy and sintering techniques. Molybdenum’s melting point is 2,617°C, boiling point = 4,612°C, and its density is 10.22 g/cm 3 .
Characteristics Given that molybdenum is located between chromium and tungsten in group 6, it chemi-
cally resembles a cross between these two partner elements. The three related elements do not occur as free elements in nature, but rather are found in minerals and ores. Their metal (elemental) radius size increases from chromium = 44 to molybdenum = 59 to tungsten = 60, which is related to their electronegativity and results in their using electrons in shells inside the outer shell during metallic bonding. This is a major characteristic of the transition of elements from metals to nonmetals.
Molybdenum oxidizes at high temperatures but not at room temperatures. It is insoluble in acids and hydroxides at room temperatures. At room temperatures, all three metals (chromium, molybdenum, and tungsten) resist atmospheric corrosion, which is one reason chromium is used to plate other metals. They also resist attacks from acids and strong alkalis, with the exception of chromium, which, unless in very pure form, will dissolve in hydrochloric acid (HCl).
AbundanceandSource Molybdenum is the 54th most abundant element on Earth. It is relatively rare and is found
in just 126 ppm in the Earth’s crust. Its major ore is molybdenite (MoS 2 ), which is mined in Colorado in the United States and is found too in Canada, Chile, China, England, Norway, Sweden, Mexico, and Australia. Moldybdenum is also found in two less important ores: wul-
fenite (PbMoO 4 ) and powellite ([Ca(MoW)O 4 ]. These ores are usually found in the same sites along with tin and tungsten ores. Molybdenite ore is very similar to graphite, and they have been mistaken for each other in the past.
History Peter Jacob Hjelm (1746–1813) is given credit for discovering molybdenum in 1781
despite the fact that his paper was not published until 1890. He followed the advice of Carl Wilhelm Scheele (1742–1786), who isolated and identified molybdenum, but incorrectly thought it was an element related to lead.
Although some reference works do give Scheele credit, most do not credit him for the discovery of either molybdenum or the other elements he “discovered,” such as oxygen and manganese.
129 Scheele did not receive credit for discovering oxygen two years before Joseph Priestley
Guide to the Elements |
(1733–1804) announced his discovery and was given the credit. Scheele’s publisher was negligent in getting his work published in time. (There is a lesson in this story for all young scientists—keep completed and accurate records of all your lab work and observations, and when you are sure of your experimental results, make sure to publish.)
The name “molybdenum” is derived from the Greek word for lead, molybdos, which stands for any black minerals that historically could be used for writing. This also explains why the Greek word plumbago or “black lead” was used for graphite.
CommonUses The high melting point of molybdenum is the major determinant of how it is used. Its chief
use is as an alloy in the manufacture of engines of automobiles. “Moly-steel” contains up to 8% molybdenum and can withstand high pressures and the relatively rapid changes of engine temperatures (e.g. cold engine to hot and back again without the metal warping and with the ability to withstand excessive expansion and contraction).
Its high melting point also makes it useful for metal electrodes in glassmaking furnaces. Molybdenum’s high resistance to electricity makes it useful in high-temperature filament wires and in the construction of parts for missiles, spacecrafts, and nuclear power generators.
Molybdenum is also used as a catalyst in petroleum refining, as a pigment for paints and printer’s ink, and as a high-temperature lubricant (molybdenum disulphide-MoS 2 ) for use by spacecraft and high-performance automobiles. In hospitals, radioisotope Mo-99, which decays into technetium-99, is given internally to cancer patients as a “radioactive cocktail.” Radioactive Tc-99 is absorbed by tissues of cancer patients, and then x-ray-like radiation is used to produce pictures of the body’s internal organs.
ExamplesofCompounds Molybdenum boride (Mo 2 B) is used to braze (weld) special metals and for noncorrosive
electrical connectors and switches. It is also used to manufacture high-speed cutting tools and noncorrosive, abrasion-resistant parts for machinery.
Molybdenum pentachloride (MoCl 5 ) is used as a brazing and soldering flux and to make fire-retardant resins. Molybdenum trioxide (MnO 3 ) is a compound used to make enamels adhere to metals. Molybdenum’s ions can exhibit lower oxidation states as follows:
Molybdenium(II) chloride (Mo 2+ + 2Cl 1- → MoCl 2 ), and