in your element Praseodymium unpaired Adrian Dingle relates how one ‘element’ that fell of the periodic table was eventually replaced by two.

f there’s a single thread that runs through elements neodymium (the new twin) and the history of the lanthanoids, it’s the praseodymium (the green twin) would Imanner in which they persistently resisted ultimately be isolated. easy distinction from one another. The Despite the death of didymium as an similarity in the chemistry of the 4f elements element, the name lives on in the specialized confounded nineteenth-century chemists glass used in the lenses of goggles worn by for decades, and it took a new analytical welders and glassblowers. The combination technique to finally tease praseodymium of praseodymium and neodymium allows from its hiding place. the filtering out of both yellow light and The story of element 59 starts with Carl infrared wavelengths, thus protecting the Gustav Mosander in 1841, and his discovery wearer’s eyes from potentially harmful of what he thought was a new element radiation without affecting their vision. in cerite. He named it didymium1, Greek Credit: imageBROKER / Alamy Stock Photo As is the same for all of the lanthanoids, for ‘twin element’, because it so closely the chemistry of praseodymium is resembled lanthanum. As it turned out, his dominated by the +​3 oxidation state — but choice of name could hardly have been a spectral fingerprints. Taken together its electronic configuration of [Xe]4f36s2 better one, although for an entirely different with the fractional precipitation and makes it a good candidate for creating reason. crystallization method and the higher oxidation states, including that of In addition to didymium (which turned development of the periodic system, they +​5. In 2016, a Pr(v) centre was reported5 + out to be fictional), by 1843, the actual led to a new surge of rare-earth discovery in PrO2 . One year later, NPr(v)O was elements cerium, lanthanum, erbium and between 1878 and 1886, yielding described, together with NPr(iv)O− — both terbium of the modern 4f series had all holmium, ytterbium, samarium, thulium, featuring a Pr≡​N triple bond and a Pr=​O been discovered. There then came a pause gadolinium, praseodymium, neodymium double bond6. in the discovery of the remainder of the and dysprosium. Another oxide of praseodymium has lanthanoids. The traditional analytical It is Carl Auer von Welsbach who is found use as a spacer layer in a potentially method of the time — the extraordinarily credited with the separation of didymium super-conducting material7. The compound

tedious fractional precipitation and into neodymium and praseodymium. Pr4Ni3O8, whose structure can be described crystallization — had reached the end of its Long before his successful fractional as three layers of a nickel oxide separated useful life as a technique able to, on its own, crystallization of the double ammonium by two layers of praseodymium oxide, distinguish between elements so similar nitrates3, though, the potential for didymium represents a chance for high-temperature to each other: they looked the same and to be something other than the singular superconductors to become a reality — and behaved in similar ways. Further discoveries substance Mosander proposed was one where praseodymium is literally at the would only come with a new impetus, which recognized by several chemists. Jean-Charles centre of things. ❐ came from both practice and theory. Galissard de Marignac, Paul-Émile Lecoq de The advent, and subsequent perfection, Boisbaudran, and Adrian Dingle of Bunsen and Kirchhoff’s spectroscopy2 Per Teodor Cleve were among the prominent Author of ‘Te Elements: An Encyclopedic throughout the early 1860s coincided with names of the time to believe didymium Tour of the Periodic Table’, teaches at Westminster, an increased understanding of patterns was not pure. Two others went further and Atlanta, GA, USA. in the properties of elements, which embarked on spectroscopic studies: Marc e-mail: [email protected] culminated with Mendeleev’s first periodic Delafontaine in 1878, and in 1882 Bohuslav Twitter: @adchempages table of 1869. At the time didymium was Brauner, who like Auer von Welsbach was a sufficiently well established as an ‘element’ student of Bunsen’s in Heidelberg. Published online: 19 April 2018 to appear on Mendeleev’s table with the But despite apparently having written https://doi.org/10.1038/s41557-018-0050-7 symbol Di — it is the only one from the a note to the Austrian newspaper Wiener original table that does not appear on the Anzeiger4, it seems that Brauner never References contemporary version. formally published his work. In 1885, 1. Mosander, C. G. Philos. Mag. 23, 241–254 (1843). 2. Kirchhof, G. & Bunsen, R. Ann. Phys. Chem. 110, 161–189 (1860). The advances in spectroscopy enabled Auer von Welsbach announced to the 3. v. Welsbach, C. A. Mon. Chem. 6, 477 (1885). chemists to finally distinguish lanthanides Vienna Academy of Sciences his successful 4. Weeks, M. E. Discovery of the Elements 7th edn, 689 (Journal of from each other thanks to their unique separation of didymium into two ‘earths’ Chemical Education, 1968). 5. Zhang, Q. et al. Angew. Chem. Int. Ed. 55, 6896–6900 (2016). (as oxides were then known): neodymia 6. Hu, S.-X. et al. Chem. Sci. 8, 4035–4043 (2017). and praseodymia, from which the 7. Zhang, J. et al. Nat. Phys. 13, 864–869 (2017).

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