Molybdenum was one of the first metals to be discovered by a modern chemist. It was found in 1781 by Swedish chemist Peter Jacob Hjelm (1746-1813). Hjelm's work on the element was not published, however, until more than a century later.
Molybdenum is a transition metal, placing it in the center of the periodic table. The periodic table is a chart that shows how chemical elements are related to one another.
Molybdenum is a hard, silvery metal with a very high melting point. It is used primarily to make alloys with other metals. An alloy is a mixture of two or more metals. The mixture has properties different from those of the individual metals. The most common alloys of molybdenum are those with steel. Molybdenum improves the strength, toughness, resistance to wear and corrosion, and ability to harden steel.
Group 6 (VIB)
Discovery and naming
The most common ore of molybdenum is called molybdenite. Molybdenite contains a compound of molybdenum and sulfur, molybdenum disulfide (MoS 2 ). Molybdenum disulfide is a soft black powder that looks like graphite. Graphite is pure carbon; it makes up the "lead" in ordinary pencils. In fact, earlier chemists thought that graphite and molybdenum disulfide were the same material.
The soft "squishy" character of molybdenum disulfide frustrated early researchers of the compound. Chemists often grind up a material before trying to dissolve it in acids or other liquids. But molybdenum disulfide cannot be ground up. The material just slides out of the way.
It was not until 1781 that Hjelm found a way to work with the compound. He discovered that it was very different from graphite. In fact, he found that it contained an entirely new element. The name chosen for the new element illustrates a further confusion. In Greek, the word for lead is molybdos. The name chosen for the new element, molybdenum, is actually the Greek word for lead!
Hjelm's work was known to his fellow chemists because of letters they had written each other. But the report of his discovery was not actually printed for all chemists to read until 1890. Between 1791 and 1891, Hjelm's research was repeated by other chemists. They confirmed what he discovered, and he is recognized today as the discoverer of molybdenum.
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.
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.
Occurrence in nature
Molybdenum never occurs free in nature. Instead, it is always part of a compound. In addition to molybdenite, it occurs commonly as the mineral wulfenite (PbMo0 4 ). Its abundance in the Earth's crust is estimated to be about 1 to 1.5 parts per million. That makes it about as common as tungsten and many of the rare earth (lanthanide) elements. About two-thirds of all the molybdenum in the world comes from Canada, Chile, China, and the United States. In the United States, molybdenum ores are found primarily in Alaska, Colorado, Idaho, Nevada, New Mexico, and Utah.
Seven naturally occurring isotopes of molybdenum exist: molybdenum-92, molybdenum-94, molybdenum-95, molybdenum-96, molybdenum-97, molybdenum-98, and molybdenum-100. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
None of the seven naturally occurring molybdenum isotopes is radioactive. However, about a dozen artificial radioactive isotopes have been produced. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
One radioactive isotope of molybdenum is commonly used in medicine, molybdenum-99m. (The "m" in this instance stands for "metastable," which means the isotope does not last very long.) This isotope is not used directly, however. Instead, it is used in hospitals to make another radioactive isotope technetium-99m. This isotope of technetium (atomic number 43) is widely used as a tracer for diagnostic studies of the brain, liver, spleen, heart, and other organs and body systems.
Molybdenum disulfide is soft and squishy.
A radioactive tracer is an isotope whose movement in the body can be followed because of the radiation it gives off. The radiation can be "traced" with special equipment held above the body. The pattern produced by the radiation allows a doctor to diagnose any unusual functioning (behavior) of the organ or body part.
Technetium-99m cannot be used for this purpose all by itself. It changes very quickly into a new isotope. Hospitals prepare molybdenum-99m first. This isotope can be stored for short periods of time. It slowly gives off radiation and changes into technetium-99m. The technetium-99m is captured as it is formed from molybdenum-99m and injected into the body for tracer studies. Because it is used to produced technetium-99m, the isotope molybdenum-99m is sometimes referred to as a "molybdenum cow.
Pure molybdenum metal can be obtained from molybdenum trioxide (Mo0
) in a variety of ways. For example, hot hydrogen can be passed over the
oxide to obtain the metal:
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.
A number of molybdenum compounds are used in industry and research. Interestingly, molybdenum disulfide is still used as a lubricant, as it was over two hundred years ago. The slippery black powder looks and behaves much like graphite. Molybdenum is used in industrial operations to reduce the friction between sliding or rolling parts. It does not break down when heated or used for very long periods of time.
Other compounds of molybdenum are used as protective coatings in materials used at high temperatures; as solders; as catalysts; as additives to animal feeds; and as pigments and dyes in glasses, ceramics, and enamels.
Molybdenum is relatively safe for humans and animals. No studies have shown it to be toxic. In fact, it is regarded as a necessary trace element for the growth of plants. A trace element is one that is needed in very small amounts for the proper growth of a plant or animal.