Stories About Chemistry
84. How Promethium Was Discovered
Strictly speaking, it was discovered many times, this element with the atomic number 61. And each time it was given a different name: illinium, florentium, cyclonium. But each time the discovery proved false, and the name of the stillborn became history.
Then scientists proved that there simply was none of the sixty-first element on Earth. Not due to some strange whim of nature by which it had deprived the Periodic System of one of its representatives, but because all the isotopes of element No. 61 are radioactive, very unstable, and had long since decayed into isotopes of its neighbouring elements.
Finally, it was produced artificially in 1945, in the course of operation of a nuclear reactor. Fission of the nuclei of uranium, the reactor “fuel”, results in a large number of fragments, these being nuclei of lighter elements. They include promethium (now the real name of this elusive element).
After pondering for a space theoretical physicists might have signed this report. But chemists would have to “feel” promethium with their hands, and to see with their eyes at least a tiny speck of the new metal, or at least its compounds.
However, it would have hardly been possible to separate more than tenths or even hundredths of a gram of element No. 61 from the mixture of fragments of uranium fission.
Is that so little? Chemists had more than once had to deal with still smaller amounts of substance, and they had been successful.
The difficulty was one of a different nature. Promethium is a rare-earth element, and we have already spoken of the resemblance in this family. Now the mixture of nuclear fragments contained a substantial amount of promethium’s closest neighbours, neodymium and samarium.
It was from them primarily that promethium had to be separated, but that was no easy job! Chemists who devoted their lives wholly to the rare earths performed a real scientific feat. It was torment—there is no other word for it—to separate the fourteen twins and to obtain each of them separately.
(The French chemist G. Urbain decided to prepare pure thulium. And he did. But it took him five years and he had to carry out over fifteen thousand monotonous and very tedious chemical operations.)
Of course, separating pure promethium was easier, but not much. It must be remembered that it is radioactive and decays rapidly. And so it might have happened that there was nothing left of it by the time it was separated.
Hence, quicker methods were needed, methods by which it would not take years, or months or even weeks to separate the lanthanides, but not more than a few hours. Chemistry knew no such methods.
Then it was that chromatography was remembered.
...Tsvet’s separating tube (it now goes by the more sedate name of chromatographic column) is filled with an adsorbent (not chalk as before, but special ion-exchange resins). A solution of salts of the rare-earth elements is passed through the resin layer. Though they resemble each other greatly, the lanthanides are not identical. Each forms a complex compound with the resin. These compounds differ in stability, and moreover, in a definite order. Lanthanum, the first in the family, makes the strongest compound; the last, lutetium, forms the weakest.
After this the resin is washed with a special solution. The drops of the solution encompass the resin grains and wash off the ions of the rare-earth elements, as it were—again in a strict sequence.
And solutions of pure rare-earth salts begin to issue from the column drop by drop: lutetium salts first, and lanthanum salts last.
It was by this method that the American scientists J. Marinsky, L. Glendenin and C. Coryell separated promethium from neodymium and samarium. And it took them only a few hours.