Chemists have, for the first time, shed light on one of the periodic table’s most elusive members, revealing the chemical properties of promethium. This rare and highly radioactive element, previously shrouded in mystery, has been coaxed into forming a compound in water, allowing researchers to observe its bonding behavior.

The chemical element of the nuclear age

Promethium, with the atomic number 61, only exists naturally in minuscule amounts. Earth’s crust contains just about half a kilogram of this element. That’s like a drop in the ocean. This scarcity also explains promethium’s discovery — which wasn’t in its natural form.

Researchers first discovered promethium by producing it in 1945 at Oak Ridge National Laboratory in Tennessee during the Manhattan Project’s plutonium enrichment program. It is named after the Greek titan Prometheus, who stole fire and brought it to humans. Nuclear power was seen as a second coming of controlled fire — or at least an invention of the same magnitude in its impact on human civilization.

However, the journey to discovering promethium was long and filled with false starts. Before its official discovery, scientists had suspected the existence of an element with atomic number 61 due to gaps in the periodic table. In the early 1900s, several claims of discovery were made, but none could be substantiated with conclusive evidence.

It wasn’t until 1945, during the height of World War II and the Manhattan Project, that promethium was finally isolated. Researchers at Oak Ridge National Laboratory in Tennessee, led by Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell, identified promethium while analyzing the byproducts of uranium fission in a nuclear reactor.

The Manhattan Project was primarily focused on developing nuclear weapons, and the discovery of new elements was a byproduct of this monumental effort. But due to the secrecy of the project, the discovery was not immediately publicized. It was only after the war ended that the findings were released, and the existence of promethium was officially confirmed.

Unstable Yet Intriguing

Promethium is now produced routinely, albeit in tiny quantities, from the radioactive decay of uranium. It can be used in simple compounds for luminous paint or nuclear batteries. However, its highly radioactive nature makes it inherently unstable. This instability complicates the formation of long-lasting compounds, which are necessary for detailed study. Furthermore, its crystal structure exerts forces on promethium’s chemical bonds, obscuring its fundamental chemistry.

Alexander Ivanov and his colleagues at Oak Ridge National Laboratory have now overcome these challenges. They managed to form a promethium compound in water. This method dampens some of the damaging effects of radioactivity and avoids the obscuring effects of crystal structures. As a result, the team could study the element’s chemistry in detail for the first time.

“Because it has no stable isotopes, promethium was the last lanthanide to be discovered and has been the most difficult to study,” said Dr. Ilja Popovs, also from Oak Ridge National Laboratory.

“There are thousands of publications on lanthanides’ chemistry without promethium. That was a glaring gap for all of science,” said Dr. Santa Jansone-Popova from Oak Ridge National Laboratory.

“Scientists have to assume most of its properties. Now we can actually measure some of them.”

Breakthrough with PyDGA

The researchers synthesized a compound called bispyrrolidine diglycolamide (PyDGA), which is known to form stable compounds with elements similar to promethium. When promethium was introduced to this molecule in a solution, it formed Pm-PyDGA, a compound that exhibits a striking bright pink color due to its electron structure.

To probe its chemical bonding, Ivanov and his team then fired X-rays at the compound and measured which frequencies it absorbed. This revealed how promethium was chemically bonded. The bond length between promethium and nearby oxygen atoms was about a quarter of a nanometre, which matched their computer simulations.

“It’s rather beautiful chemistry, and to see the delicate pink color of this complex is a real joy,” Andrea Sella at University College London told New Scientist.

Information about promethium’s bonding behavior will help improve processes for producing purer samples in larger quantities from radioactive waste. This could lead to the design of new medical compounds, such as those used in radioactive imaging or cancer treatment. “This kind of fundamental information could help us to drive new technologies,” says Ivanov.

Moreover, this research may impact the development of nuclear batteries and luminous materials. The unique properties of promethium compounds could lead to more efficient and longer-lasting sources of energy and light, which are critical in remote or extreme environments where traditional power sources fail.

The findings appeared in the journal Nature.