Molybdenum – facts and properties
- Symbol
- Mo
- Atomic number
- 42
- Atomic mass
- 95.95 u
- Period
- 5
- Group
- 6
- Category
- transition metal
- Electron configuration
- [Kr] 4d⁵ 5s¹
- Electronegativity
- 2.16
- Atomic radius
- 190 pm
- Valence electrons
- 6
- State at standard conditions
- solid
- Melting point
- 2623 °C
- Boiling point
- 4639 °C
- Density
- 10.2 g/cm³
- Discovered
- 1778
Molybdenum is a silver-grey, hard transition metal with atomic number 42 and one of the highest melting points of all metals. Its name comes from the Greek word molybdos for lead, because the mineral was long mistaken for galena. Molybdenum hardens steel and makes it resistant to heat and corrosion, its disulfide lubricates like graphite, and as a trace element it is indispensable to plants, animals and people.
Position in the periodic table
Occurrence and extraction
Molybdenum occurs in nature only in compounds. The most important ore is molybdenite, a molybdenum disulfide with the formula MoS₂. It forms soft, lead-grey flakes that look deceptively like graphite or galena. That confusion is built into the name: the Greek molybdaina meant lead ore. From it came molybdaena in early modern Latin, a collective name for soft minerals that mark, such as graphite, galena and molybdenite. Another mineral is wulfenite, a yellow to orange lead molybdate.
The Swedish chemist Carl Wilhelm Scheele recognised the difference. In 1778 he showed that molybdenite is neither graphite nor galena but contains an acid of its own. A few years later his countryman Peter Jacob Hjelm produced the metal from it.
Today a large share is a by-product of copper mining. The ore is crushed, concentrated and roasted in air, which forms molybdenum trioxide, and this is reduced with hydrogen to metal powder. Large amounts come from China, the United States, Chile and Peru.
Properties and reactions
Molybdenum is in period 5 and group 6, below chromium. Its electron configuration is [Kr] 4d⁵ 5s¹ and its electronegativity is 2.16. The metal has a density of 10.2 g/cm³, melts at 2623 °C and boils at 4639 °C. It is hard, expands little when heated and conducts electricity and heat well.
At room temperature molybdenum is stable in air and resists most acids; only strongly oxidising mixtures such as hot nitric acid or aqua regia dissolve it. At about red heat it burns to molybdenum trioxide, a white to yellowish powder that evaporates at high temperature. In compounds it appears in several oxidation states, most often plus six.
The structure of molybdenum disulfide is special. Its atoms lie in thin layers that cling to one another only weakly and slide over each other easily. That is why the powder feels smooth and greasy and works as a dry lubricant.
Uses in everyday life and technology
Most molybdenum goes into steelmaking. Small additions make steel harder, tougher and more resistant to heat and corrosion. Stainless steels with molybdenum withstand chloride solutions such as salt water better, so they are used for ship parts, kitchen equipment and chemical plants. Tool steel, pipelines and engine parts contain molybdenum too.
Molybdenum disulfide serves as an additive in greases and oils and as a dry film where ordinary lubricants fail, for example under very high pressure or in vacuum. In the oil industry, molybdenum catalysts are important for removing sulfur from petrol and diesel. The metal itself is found in furnace heating elements, in the support wires of light bulbs and in electrodes for glass melting.
Molybdenum also plays a key role in biology. The enzyme nitrogenase, with which bacteria convert nitrogen from the air into ammonia, normally contains an iron-molybdenum centre. Without molybdenum the nodule bacteria on the roots of peas and beans could not fix nitrogen. Plants need it to make use of nitrate, and people require tiny amounts for several enzymes, for instance in the breakdown of purines and sulfur compounds. Deficiency is rare, and extra supply is unnecessary.
Molecules containing this element
- Molybdenum(IV) sulfide MoS2
- Molybdenum(VI) oxide MoO3
- Lead(II) molybdate PbMoO4
Quick question
What is the atomic number of molybdenum (Mo)?
FAQ
Why is molybdenum called molybdenum?
The word comes from ancient Greek molybdos, lead. The mineral molybdenite looks like galena and graphite and marks like lead. It was long taken for a lead ore until Scheele recognised that it contains an element of its own.
What is molybdenum used for?
Mainly as an additive in steel, which it makes harder and more resistant to heat and corrosion. It is also used in lubricants containing molybdenum disulfide, in catalysts for oil processing and as a trace element that enzymes such as nitrogenase need.
Is molybdenum important for humans?
Yes, in very small amounts. It is part of several enzymes, for example in the breakdown of sulfur compounds and purines. A normal, varied diet covers the need, and deficiency is very rare.