Molybdenum (Mo)

• Element Symbol: Mo
• Atomic Number: 42
• Atomic Mass: 95.94
• Group # in Periodic Table: 6
• Group Name: Transition metals
• Period in Periodic Table: 5
• Block of Periodic Table: d-block
• Discovered by: Carl Wilhelm Scheele (1778)

Molybdenum is a silvery-white metal with an extremely high melting point. Its chemical symbol is Mo, and its atomic number is 42, meaning it is a period 5 transition metal in the periodic table. This element does not react with oxygen or water at room temperature. It also resists corrosion at normal temperatures. Molybdenum is one of the five major refractory metals, along with tungsten, tantalum, rhenium, and niobium. A refractory metal is one that has a very high resistance to heat and wear. Molybdenum metal and its compounds have numerous uses. Biologically, in very small trace amounts, this element is an essential one for animals because it can be found in approximately twenty enzymes needed for metabolism.

Molybdenum minerals have been known throughout history, but the element was discovered in 1778 by Carl Wilhelm Scheele, a Swedish chemist. Another Swedish chemist, Peter Jacob Hjelm, first isolated molybdenum metal in 1781 by extracting it from its primary ore, molybdenite, using carbon and linseed oil.

Until molybdenum’s formal discovery, molybdenite was often confused with graphite because it was thought to contain lead. Its name, from the ancient Greek word molybdos, even means "lead." This confusion was due to the fact that molybdenite is a soft black mineral just like graphite. Today, this mineral is also known as molybdenum disulfide (MoS₂).

Physical Properties

In its pure form molybdenum is a shiny, silvery-white metal with an extremely high melting point of 2623 degrees Celsius (°C). Only five other naturally occurring elements have higher melting points. They are, in order of increasing melting point, tantalum, osmium, rhenium, tungsten, and carbon. This transition metal is dense and hard, with a density of 10.28 grams per cubic centimeter (g/cm³) and a Mohs hardness rating of 5.5. Its standard state at 298 kelvins (K) is solid. Molybdenum is also resistant to corrosion; it only starts to rust at 300 °C. This means that it has one of the lowest coefficients of thermal expansion among commercially used metals. Thermal expansion is the tendency of matter to change in volume in response to temperature changes through heating. In other words molybdenum is very useful commercially because it is hard and dense and because it does not rust easily. In addition, at 20 °C its thermal conductivity is 142 watts per meter-kelvin (W/m·K).

Chemical Properties

Molybdenum is a transition metal with an electronegativity of 2.16 on the Pauling scale, which means that it can easily combine with other elements to form compounds. The elements that molybdenum most often combines with are carbon, oxygen, chlorine, sulfur, and fluorine. Molybdenum does not react visibly with oxygen at room temperature. It does have several oxidation states, ranging from −2 to +6, with the most stable ones being +4 and +6. Chemically, molybdenum and its various compounds are very similar to tungsten.

There are thirty-five known isotopes of this element, ranging in atomic mass from 83 to 117, of which seven occur naturally. These isotopes have atomic masses of 92, 94, 95, 96, 97, 98, and 100. Of these seven naturally occurring isotopes, only molybdenum-100 is unstable, with a half-life of about 7.8 ×10¹⁸ years. However, molybdenum-98 is the most abundant isotope, making up 24.14 percent of all molybdenum. Molybdenum-98’s half-life is approximately 1.0 × 10¹⁴ years. Comparatively, molybdenum isotopes 111 to 117 have half-lives of around 150 nanoseconds.

Applications

Molybdenum is the fifty-fourth most abundant element in Earth’s crust (1.1. parts per million) and the twenty-fifth most abundant in Earth’s oceans (10 parts per billion); it also the forty-second most abundant element in the universe, with a concentration of 9 parts per billion. This comparative scarcity in Earth’s crust is offset by the element’s concentrations in a number of water-insoluble ores, such as molybdenum disulfide (MoS₂), or as it is more commonly known, molybdenite. Molybdenum can also be found in two other mineral ores. The first is wulfenite (PbMoO₄) or lead molybdate. The other is powellite (CaMoO₄) or calcium molybdate. Although there are three potential sources, molybdenum is mined from its principal ore, molybdenite. The element has also been recovered as a by-product as a result of copper and tungsten mining.

A total of 250,000 metric tons (t) of molybdenum were produced in 2011. The largest producers were China (94,000 t), the United States (64,000 t), Chile (38,000 t), Peru (18,000 t), and Mexico (12,000 t). The total estimated reserves of this element are 10 million metric tons. Molybdenum production is almost evenly split between North America, South America, and China. Europe and the rest of Asia—mainly Armenia, Russia, Iran, and Mongolia—produce the remainder of molybdenum currently available. Historically, the Knaben mine in southern Norway, opened in 1885, was the first dedicated molybdenum mine. Today, the largest molybdenum mines are located in Colorado, Utah, British Columbia, and Chuquicamata, Chile.

Molybdenum has a number of commercial applications. However, it is used in four main ways. First, it is used in small quantities to harden steel and other alloys. Second, because of its strength and resistance to thermal expansion, it is often used in nuclear power plants and aircraft engines. Third, molybdenum is used as glass furnace electrodes because of its extremely high melting point. Finally, it is used in the petroleum industry as a catalyst in the removal of organic sulfur compounds from coal and liquid gas.

Molybdenum also has a few biological uses. Although it is toxic in anything other than small concentrations, the element is an essential trace element for plant and animal survival. In plants molybdenum is used by nitrogen-fixing bacteria to convert nitrogen gas in the air into nitrates vital for plant growth. This process uses the enzyme nitrogenase, of which molybdenum is a component. In animals the element is found in approximately twenty enzymes needed for metabolism. Specifically, in mammals, four molybdenum-dependent enzymes are known to be used by the mitochondria in cells to covert food into necessary energy. Interestingly, the human body contains an average of 0.07 milligrams of molybdenum per kilogram of weight and can be found in high concentrations in the kidneys and liver. It is also found in smaller concentrations in the backbone and teeth.

Bibliography

Cottrell, T. L. The Strengths of Chemical Bonds. New York: Academic, 1954. Print.

Coursey, J. S., et al. Atomic Weights and Isotopic Compositions with Relative Atomic Masses. NIST Physical Measurement Laboratory. Natl. Inst. of Standards and Technology, 30 Sept. 2015. Web. 13 Nov. 2015.

Emsley, John. Nature’s Building Blocks: An A–Z Guide to the Elements. 2nd ed. New York: Oxford UP, 2011. Print.

Haynes, William M., ed. CRC Handbook of Chemistry and Physics. 95th ed. Boca Raton: CRC, 2014. Print.

"It’s Elemental: The Periodic Table of Elements." Science Education at Jefferson Lab. Thomas Jefferson Natl. Accelerator Facility, n.d. Web. 22 Sept. 2015.

Kaye & Laby Tables of Physical & Chemical Constants. Natl. Physical Laboratory, 2015. Web. 22 Sept. 2015.

"Molybdenum Elemental Facts." Chemicool. Chemicool.com, 17 Oct. 2012. Web. 22 Sept. 2015.