Molybdenum

Molybdenum is a silvery group 6 transition metal sitting between chromium and tungsten. Like chromium, it has a half-filled 4d subshell ([Kr] 4d5 5s1), and those six valence electrons form strong metallic bonds, giving molybdenum a melting point of 2,623 °C, among the highest of any element. It shows oxidation states from −2 to +6, with +6 the most stable, as in molybdenum trioxide and the molybdate ion, MoO4^2−. In ores it usually occurs as molybdenite, MoS2, a soft, greasy, lead-grey sulfide that early miners confused with graphite and lead ore.

Most mined molybdenum ends up in alloy steels. A few tenths of a percent raises strength at high temperature and resists pitting by chloride, which is why it appears in stainless steels, drill bits and chromoly bicycle frames. Molybdenite itself is valuable: its sulfur-molybdenum layers slide over each other, making it a dry lubricant, and cobalt-molybdenum sulfide catalysts remove sulfur from petroleum fuels. Molybdenum is also the only element in the second transition series that most living things need. It sits at the active site of enzymes that handle nitrogen, sulfur and purines.

Discovery

For centuries molybdenite, graphite and galena were all called 'molybdena', from the Greek molybdos, meaning lead. In 1778 Carl Wilhelm Scheele showed that molybdenite was a distinct substance by converting it into a white acidic oxide. Lacking a suitable furnace, he passed the problem to Peter Jacob Hjelm, who in 1781 reduced the oxide with carbon and linseed oil to obtain the impure metal. The element kept the old name. Molybdenum saw little use until the First World War, when Germany added it to armour plate and gun steel.