Molybdenum

Molybdenum is a silvery refractory metal that most people never encounter directly, yet it is inside almost every stainless steel tool, structural girder and engineering component built to withstand heat or corrosion. It is also the only element in the second transition-metal row to be essential for life, sitting at the heart of enzymes that fix atmospheric nitrogen.

• Atomic Number4242 protons, 42 electrons
• Atomic Mass95.95 u42× heavier than hydrogen
• State at Room TempSolidSolid
• Density10.2 g/cm³
• Melting / Boiling2622.8°C / 4638.9°C
• Discovered1778

What is Molybdenum?

Molybdenum is a transition metal in Group 6, sitting below chromium. With 42 protons and a melting point of 2,623°C, it is one of the most heat-resistant metals known. It forms compounds in oxidation states from −2 to +6, with +4 and +6 being most common. In nature it is always found combined with oxygen or sulfur. Molybdenum disulfide (MoS₂) is a naturally slippery material used as a dry lubricant.

Molybdenum gets its name from the ancient Greek word molybdos meaning lead, because the mineral molybdenite (MoS₂) was once confused with galena (lead sulfide), both are shiny, dark and soft. Carl Wilhelm Scheele distinguished them in 1778 and identified the new element. Peter Jacob Hjelm isolated the metal in 1781. The symbol Mo comes from the Latin Molybdaenum.

Fact Molybdenum has the sixth-highest melting point of all elements at 2,623°C. It was this extreme heat-resistance that led to its use in the filament supports of incandescent light bulbs and in the X-ray tubes used in medical imaging.

Where you find Molybdenum

In space

Molybdenum is found in the Sun and other stars, produced in stellar nucleosynthesis.

On Earth

Molybdenum is found mainly in the ore mineral molybdenite.

• Molybdenite (MoS₂). The primary ore, found in large porphyry copper deposits. China, the USA, Chile and Peru are the main producers.
• By-product of copper mining. Much molybdenum is recovered as a by-product when processing copper ore.

How we use Molybdenum

• High-strength steels.. About 80% of molybdenum goes into steel alloys. It greatly improves strength, hardness, weldability and corrosion resistance at high temperatures.
• Superalloys.. Molybdenum-containing nickel superalloys are used in jet engines and gas turbines.
• Dry lubricant.. MoS₂ powder is used as a dry lubricant in conditions where oils would burn away, space mechanisms, high-temperature bearings and gun mechanisms.
• Biology.. Molybdenum is an essential trace element for plants and animals. The enzyme nitrogenase, which converts atmospheric nitrogen into ammonia, has molybdenum at its active site.

Did you know? The enzyme nitrogenase, which allows bacteria to convert nitrogen gas from the air into ammonia that plants can absorb, contains a cluster of iron and molybdenum atoms at its active site. Without molybdenum, biological nitrogen fixation would be impossible and most of the world's plants would starve.

How it was discovered

Molybdenite was identified as containing a new oxide by Scheele in 1778. Peter Jacob Hjelm isolated impure molybdenum metal in 1781 by reducing the oxide with carbon. Pure molybdenum was not obtained for another century.

Deeper dive: molybdenum chemistry and applications

Molybdenum's importance in steel arises from several effects: it increases hardenability (ability to be hardened by heat treatment), prevents temper brittleness and increases strength at elevated temperatures. High-speed steel, which retains hardness at the temperatures generated by machining, invariably contains molybdenum alongside tungsten, vanadium and chromium. Tool steels, stainless steels and alloy steels all benefit from molybdenum additions.

In biology, molybdenum cofactors (Moco) are present in approx. 50 known enzymes, all of which catalyse reactions involving the transfer of oxygen atoms. The most important are nitrogenase (nitrogen fixation), nitrate reductase (assimilation of nitrate by plants) and xanthine oxidase (breakdown of purines in animals). Molybdenum is one of the heaviest elements required for life.

Moving to 43 protons on the periodic table brings us to Technetium.