Molybdenum Mo

Discovery

For centuries the soft black mineral molybdenite (molybdenum disulfide) was confused with graphite and with lead ores, which it resembles. All of them were lumped together under names such as molybdaena.

In 1778 the Swedish chemist Carl Wilhelm Scheele treated molybdenite with nitric acid and obtained a new oxide that was neither graphite nor lead, concluding that it contained an unknown element.

At Scheele's suggestion, Peter Jacob Hjelm reduced the oxide with carbon in 1781 and isolated the metal. In the early twentieth century molybdenum steels were developed, and demand rose sharply in armaments and machinery.

Origin of the name

The name comes from the Greek molybdos, "lead", a reminder of its long confusion with lead minerals. The symbol is Mo.

Main uses

• Alloy steels: most molybdenum goes into steel. Adding 2–3% to stainless steel, as in grade 316, greatly improves resistance to corrosion by chloride in seawater. It also strengthens high-speed tool steels and pipeline steels.
• Lubricants: molybdenum disulfide has a layered structure that slides easily, making it a solid lubricant for high loads and temperatures.
• Catalysts: molybdenum-based catalysts remove sulfur from crude oil in refineries through hydrodesulfurisation.
• Electronics and medicine: it serves as the target in mammography X-ray tubes, as wiring in displays and as heating elements in high-temperature furnaces. Molybdenum-99 is the source of medical technetium-99m.

Isotopes

Natural molybdenum consists of seven isotopes, molybdenum-92, -94, -95, -96, -97, -98 and -100, with molybdenum-98 the most abundant. Molybdenum-100 decays by double beta decay with an extremely long half-life, so it is effectively stable.

Artificial molybdenum-99, with a half-life of about 66 hours, underpins nuclear medicine worldwide. Inside generators shipped to hospitals, it decays continuously into technetium-99m, which staff draw off whenever they need it for a scan.

In everyday life

Molybdenum comes from molybdenite, mined for its own sake and also recovered in large quantities as a by-product of copper mining. You meet it in stainless kitchenware, engine oil additives and cutting tools.

In the body, enzymes such as xanthine oxidase and sulfite oxidase require molybdenum. Legumes, grains and nuts are rich sources, so deficiency is very rare. Bacteria in the root nodules of legumes use an iron–molybdenum enzyme, nitrogenase, to turn atmospheric nitrogen into ammonia. Very high intake can interfere with copper metabolism, but ordinary diets pose no problem.