Molybdenum is a transition metal, distinguished by the chemical symbol Mo and atomic number 42. Its discovery was a protracted process stretching over centuries due to the historical challenge of differentiating its ore from other minerals. The element’s unique properties, such as a high melting point, were obscured for generations by its physical resemblance to common materials. The eventual identification of molybdenum required meticulous chemical analysis and the development of new laboratory techniques to isolate the pure metallic form.
Mistaking Molybdenum for Other Minerals
The mineral from which molybdenum is primarily extracted, molybdenite (molybdenum sulfide, MoSâ‚‚), was historically confused with two other common substances. Its soft, greasy, silver-gray appearance and tendency to leave a dark mark caused it to be mistaken for graphite (a form of carbon). Molybdenite also looks similar to the common lead ore, galena (lead sulfide).
The name “molybdenum” itself reflects this ancient misidentification, deriving from the Ancient Greek word molybdos, meaning “lead.” For centuries, molybdos was used collectively for any lead-like material used for marking or writing, including lead, galena, and the element’s ore. This confusion had to be overcome before the element’s distinct chemical identity could be established in the late 18th century.
The Scientists Who Isolated the Element
The process of discovery began in 1778 with the Swedish chemist Carl Wilhelm Scheele, who proved that molybdenite was not carbon or lead ore. He achieved this by decomposing a sample of molybdenite in hot nitric acid, producing a white powder he identified as the oxide of a new element. Scheele established that molybdenite was a sulfide compound of a unique substance, which he named molybdenum after the mineral.
Scheele was unable to reduce this oxide to its pure metallic form, which remained a significant challenge. The final step was accomplished by another Swedish chemist, Peter Jacob Hjelm, to whom Scheele sent a sample of the isolated oxide. Hjelm successfully reduced the molybdic acid using carbon and linseed oil in 1781, obtaining a dark, impure powder of the metallic element. The official date of the element’s isolation is recognized as 1781, with Hjelm credited as the first to produce the metal itself.
Defining the New Element
The successful isolation of the metal allowed the scientific community to chemically characterize molybdenum, confirming its distinct nature. This work definitively separated it from lead and graphite, ending centuries of confusion stemming from the physical similarity of their ores.
Molybdenum’s unique properties were then cataloged, including its high melting point, which explained the difficulty Hjelm faced in its isolation. The metal remained primarily a laboratory curiosity for nearly a century. This was because the high temperatures and specialized techniques required for its commercial extraction and processing were not yet available.