Molybdenum is the chemical element with symbol Mo and atomic number 42. It is a silvery transition metal with a very high melting point, excellent high-temperature strength and important roles in steel alloys, catalysts, lubricants and biological enzymes.
Molybdenum quick facts
| Property | Value |
| Symbol | Mo |
| Atomic number | 42 |
| Relative atomic mass | 95.95 |
| Group | 6 |
| Period | 5 |
| Block | d-block |
| Density | 10.2 g/cm³ |
| Melting point | 2622°C |
| Boiling point | 4639°C |
| Electron configuration | [Kr] 4d⁵ 5s¹ |
| Common oxidation states | +4, +6 |
| CAS number | 7439-98-7 |
| Key natural isotopes | ⁹⁵Mo, ⁹⁶Mo, ⁹⁸Mo |
Position and chemistry
Molybdenum is in group 6, period 5 and the d-block. Its electron configuration is [Kr] 4d⁵ 5s¹. Molybdenum forms several oxidation states, with +4 and +6 particularly important in oxides, sulfides and molybdate compounds.
See the complete CleverlySmart periodic table.
Physical properties
Molybdenum is hard and refractory, meaning it withstands very high temperatures. Its melting point is above 2600°C. It also has a relatively low coefficient of thermal expansion for a metal, making it useful where dimensional stability at high temperature matters.
Important molybdenum compounds
- Molybdenum disulfide (MoS₂): a layered solid used as a dry lubricant and increasingly important in two-dimensional materials research.
- Molybdenum trioxide (MoO₃): an important oxide used in catalysts, pigments and chemical processing.
- Molybdates: salts containing molybdenum(VI), used in corrosion inhibition, pigments, catalysts and analytical chemistry.
Occurrence and production
The principal molybdenum ore is molybdenite (MoS₂). Molybdenum can be mined from dedicated deposits or recovered as a by-product of copper mining. Concentrated molybdenite is roasted to an oxide and then processed into ferromolybdenum, metal powder or high-purity compounds.
Uses of molybdenum
- Steel alloys: molybdenum improves strength, hardenability, corrosion resistance and performance at elevated temperatures.
- High-temperature components: used in furnace parts, electrodes and specialised heating elements.
- Lubricants: MoS₂ works under high loads, vacuum and temperature conditions where oils may fail.
- Petroleum catalysts: molybdenum sulfide-based catalysts help remove sulfur from fuels.
- Electronics and materials: molybdenum and MoS₂ are used in thin films, contacts and emerging semiconductor devices.
Biological role
Molybdenum is an essential trace element for humans, animals, plants and many microorganisms. It occurs in molybdenum cofactors used by enzymes involved in sulfur metabolism and other reactions. In nitrogen-fixing bacteria, molybdenum is a key part of the most common form of nitrogenase.
Molybdenum isotopes
Natural molybdenum contains seven naturally occurring isotopes, including ⁹²Mo, ⁹⁴Mo, ⁹⁵Mo, ⁹⁶Mo, ⁹⁷Mo, ⁹⁸Mo and ¹⁰⁰Mo. Molybdenum-100 is very long-lived and is studied in double-beta-decay and neutrino experiments.
For broader isotope data, see the CleverlySmart isotope table.
Safety
Molybdenum is essential only in small amounts. Occupational exposure to metal powders, welding fumes or soluble compounds should be controlled. Toxicity depends on dose and chemical form.
Discovery and name
For centuries molybdenite was confused with graphite and lead ores. Carl Wilhelm Scheele showed in 1778 that it contained a new element, and Peter Jacob Hjelm isolated molybdenum metal in 1781. The name derives from the Greek molybdos, meaning lead.
Frequently asked questions
Why is molybdenum added to steel?
It improves strength, wear resistance, hardenability and performance in hot or corrosive environments.
Is molybdenum essential to life?
Yes. It is required in tiny amounts by enzymes in humans and many other organisms.
Molybdenum in steels and catalysts
Molybdenum raises the high-temperature strength, hardenability and corrosion resistance of many steels. It is especially useful in pressure vessels, pipelines, stainless steels and tool steels that must retain strength under heat or chemical attack.
Molybdenum compounds are also major industrial catalysts. Molybdenum sulfide-based catalysts help remove sulfur from petroleum streams during hydrodesulfurisation, reducing sulfur emissions from fuels.
Molybdenum disulfide and biology
Molybdenum disulfide (MoS₂) has a layered crystal structure whose sheets slide over one another easily, making it a useful solid lubricant in vacuum, aerospace and high-temperature applications. Atomically thin MoS₂ is also studied as a two-dimensional semiconductor.
Molybdenum is an essential trace nutrient because several enzymes use molybdenum-containing cofactors to carry out redox reactions, including steps in sulfur and nitrogen metabolism.
Frequently asked questions
Why is molybdenum added to stainless steel?
It improves resistance to pitting and crevice corrosion, especially in chloride-containing environments.
What makes MoS₂ a good lubricant?
Its strongly bonded atomic layers are weakly attracted to neighbouring layers, allowing them to slide with relatively low friction.
