The Periodic Table of the Elements is a testament to the many millennia of human exploration of the chemical world. However, not everything is known about the elements that appear in its colored and meticulously arranged rows and columns. One of them is the promethium.
First discovered 80 years ago in 1945, promethium is a lanthanide (one of a series of 15 metallic chemical elements also known as rare earth metals) with the atomic number 61, and in the eight decades following its discovery, many of its chemical properties remained a mystery. This did not prevent its use – traces of the element can be found in all types of materials, from smartphone screens to nuclear batteries – but its study has proven difficult. This is because it is an extremely rare element that breaks down into other elements, meaning that promethium can only be obtained from fission.
Scientists at Oak Ridge National Laboratory, a descendant of the original laboratory that discovered the element in 1945, launched a new process last year that created a pure sample of Promethium-147, an isotope of promethium. Once this sample was combined with a ligand to form a stable complex in water, the team was finally able to analyze the binding properties of promethium using X-ray spectroscopy. The results of the study were published last week in the journal Nature.
“Because it has no stable isotopes, promethium was the last lanthanide to be discovered and the most difficult to study,” said Ilja Popovs of ORNL and co-author of the study. “Anything we could call a modern marvel of technology would include, in one form or another, these rare earth elements… we’re adding the missing link.”
To get closer to the element promethium, researchers first created a compound known as bispyrrolidine diglycolamide (PyDGA). When combined with promethium, the electronic structure of the resulting Pm-PyDGA created a pinkish hue, but more importantly it allowed the scientists to fire X-rays and measure the absorbed frequencies, which gave clues to the chemical bonds of promethium.
Understanding promethium and its binding properties will help ORNL produce greater quantities of this rare earth metal and improve methods for separating it from other lanthanides. That’s because the team successfully demonstrated a phenomenon known as “lanthanide shrinkage,” which explains how as the number of atoms in the lanthanide series increases, the radii of the ions decrease, according to ORNL. This creates a specific chemical and electronic signature, and ORNL scientists recorded a clear “promethium signal,” which will help understand the trend in other rare earth metals.
“You can’t use all of these lanthanides as a mixture in modern advanced technologies, because you have to separate them first,” Santa Jansone-Popova, “This is where shrinkage becomes very important; it basically allows us to separate them, which is still a quite a difficult task.”
So while the Periodic Table of the Elements is a story about humanity’s chemical ingenuity, it is also a scientific story that continues to unfold in laboratories around the world.
Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff from him at Gizmodo and Paste if you look hard enough.
