Element 61—Promethium

The last of our seven elements to be isolated was element 61, which is also the only rare earth among the seven. The problem with rare earths, which are 15 or even 17 in number depending on precisely how they are counted, is that they are extremely similar to each other and as a result are very difficult to separate. When the periodic table was first discovered in the 1860s only two or three rare earths even existed. As more of them turned up it became increasingly difficult to place them in the periodic system. Just like with all the other seven elements in our story, there were many false claims to its discovery. Moreover, the early claims must have seemed very plausible at the time because they appeared to draw support from X-ray evidence and Moseley’s law. Just like the priority dispute involving hafnium that took place in the early 1920s, the case of element 61 also involved an international controversy. This time one cannot entirely blame the aftermath of the Great War, as the two opponents consisted of Italians and Americans, with much of the scientific chicanery taking place, as was usual for the time, in the pages of London’s Nature magazine. But even though both sides of the priority dispute appealed to X-ray data and Moseley’s law, it turned out that neither side was right. In their own way, each side was working in complete delusion, since element 61 is highly radioactive and unstable, does not occur naturally on Earth, and could only be isolated in minute quantities by artificial means when such methods became sufficiently developed in the 1940s. Let us start at the beginning. In 1902, the Bohemian rare earth chemist Bohuslav Brauner was the first to suggest that an element lying precisely between neodymium and samarium remained to be discovered. He gave talks in his native Bohemia and published articles in some fairly obscure journals, all of which meant that few chemists in the wider arena became aware of his work.