Molybdenum

Molybdenum is a chemical element, represented by the symbol Mo and carrying atomic number 42. Its name comes from the Ancient Greek word *mólybdos*, which means lead, a confusion that arose because its ores were often mistaken for lead ores. While molybdenum minerals have been known for centuries, the element was first recognized as a distinct substance in 1778 by Carl Wilhelm Scheele, and the pure metal was isolated three years later, in 1781, by Peter Jacob Hjelm.

On Earth, molybdenum never appears as a free metal; it exists only in oxidized forms within its minerals. In its pure state, it is a silvery metal with a grey tint and boasts the sixth-highest melting point among all elements. This property, along with its tendency to form hard, stable carbides when combined with other metals, makes it highly valuable in alloys. About 80% of the world's molybdenum production goes into steel alloys, including high-strength varieties and superalloys.

Most molybdenum compounds do not dissolve easily in water. However, when molybdenum-bearing minerals are heated in the presence of oxygen and water, they form the molybdate ion (MoO₄²⁻), which creates quite soluble salts. Industrially, roughly 14% of global molybdenum output is used in pigments and catalysts.

In biology, molybdenum is crucial. Molybdenum-containing enzymes are the most common bacterial catalysts for breaking the chemical bond in atmospheric nitrogen during biological nitrogen fixation. Over 50 molybdenum enzymes are known across bacteria, plants, and animals, though only those in bacteria and cyanobacteria are involved in nitrogen fixation. Most nitrogenases contain an iron-molybdenum cofactor (FeMoco), which likely holds molybdenum in either the +3 or +4 oxidation state. In contrast, all other molybdenum-bearing enzymes bind molybdenum in the +6 or +4 state with a molecule called molybdopterin. Molybdenum is an essential element for all higher eukaryotes, including humans. One notable organism, the sponge *Theonella conica*, is known for hyperaccumulating molybdenum.

**Characteristics**

**Physical properties** Pure molybdenum is a silvery-grey metal with a Mohs hardness of 5.5 and a standard atomic weight of 95.95 g/mol. Its melting point is 2,623 °C (4,753 °F), the sixth highest among naturally occurring elements—only tantalum, osmium, rhenium, tungsten, and carbon melt at higher temperatures. It also has one of the lowest coefficients of thermal expansion among commercially used metals.

**Chemical properties** Molybdenum is a transition metal with an electronegativity of 2.16 on the Pauling scale. At room temperature, it does not visibly react with oxygen or water, but it is attacked by halogens and hydrogen peroxide. Weak oxidation begins at 300 °C (572 °F), and bulk oxidation occurs above 600 °C, producing molybdenum trioxide. Like many heavier transition metals, it rarely forms a cation in water, though the Mo³⁺ cation can be created under carefully controlled conditions. In its gaseous state, molybdenum exists as diatomic Mo₂ molecules. This molecule is a singlet, with two unpaired electrons in bonding orbitals plus five conventional bonds, resulting in a sextuple bond.

**Isotopes** There are 39 known isotopes of molybdenum, with atomic masses from 81 to 119, along with 13 metastable nuclear isomers. Seven isotopes occur naturally, with atomic masses of 92, 94, 95, 96, 97, 98, and 100. Molybdenum-98 is the most abundant, making up 24.14% of natural molybdenum. Only molybdenum-100 is unstable; it decays via double beta decay into ruthenium-100, with a half-life of 7.07×10¹⁸ years. All synthetic isotopes of molybdenum decay into isotopes of niobium, technetium, or zirconium. The most stable synthetic isotope is ⁹³Mo, which has a half-life of 4,839 years and decays by electron capture into stable niobium. The most common isotopic application involves molybdenum-99, a fission product. It is the parent radioisotope of technetium-99m, a short-lived gamma-emitting nuclear isomer used in medical imaging.

**Redox buffer in the irradiated fuel matrix** In spent nuclear fuel, molybdenum acts as a redox buffer. ⁹⁹Mo is one of the most abundant fission products, with a fission yield of 6.1%, close to that of ¹³⁵Xe (6.33%). Molybdenum plays a critical role in nuclear fuel chemistry because it affects the fuel's oxygen fugacity. Molybdenum produced by nuclear fission in the fuel matrix inhibits the oxidation of uranium dioxide.

**Compounds** Molybdenum forms compounds in oxidation states from −4 and −2 up to +6. Higher oxidation states are most relevant to its natural occurrence and biological roles; mid-level states are often found in metal clusters, while very low states are typical of organomolybdenum compounds. The chemistry of molybdenum and tungsten is very similar. For instance, molybdenum(III) compounds are rare, unlike the common chromium(III) compounds. The highest oxidation state appears in molybdenum(VI) oxide (MoO₃), while the normal sulfur compound is molybdenum disulfide (MoS₂). Commercially, the most important compounds are molybdenum disulfide and molybdenum trioxide. The black disulfide is the main mineral; it is roasted in air to produce the trioxide: 2 MoS₂ + 7 O₂ → 2 MoO₃ + 4 SO₂. The trioxide, which is volatile at high temperatures, is the precursor to nearly all other molybdenum compounds and alloys. Molybdenum has several oxidation states, with +4 and +6 being the most stable. Molybdenum(VI) oxide dissolves in strong alkaline water, forming molybdates (MoO₄²⁻). Molybdates are weaker oxidants than chromates. At lower pH, they condense into structurally complex oxyanions, such as [Mo₇O₂₄]⁶⁻ and [Mo₈O₂₆]⁴⁻. Polymolybdates can incorporate other ions, forming polyoxometalates, including the dark-blue phosphorus-containing heteropolymolybdate.