Unveiling the Secrets of Promethium: A Breakthrough in Lanthanide Chemistry

Promethium, a rare earth element discovered in 1945 at Clinton Laboratories, has remained somewhat of a mystery despite its applications in medicine and nuclear technology. 

This element, symbolized by Pm and atomic number 61, is named after the mythological Prometheus who brought fire to humanity.

However, unlike other lanthanides, promethium has no stable isotopes, making it difficult to study and hindering a complete understanding of its properties. "The goal," explained Dr. Alex Ivanov, a researcher at Oak Ridge National Laboratory, "was to unlock the secrets of this rare element and gain new knowledge."

Dr. Ivanov and his team achieved a breakthrough by creating a chemical complex of promethium, allowing its characterization in a solution for the first time. This meticulous feat exposed crucial details about the element.

"Due to the lack of stable isotopes," said Dr. Ilja Popovs, another researcher at Oak Ridge National Laboratory, "promethium was the last lanthanide discovered and the most challenging to study. Countless publications exist on lanthanide chemistry, yet promethium remained a glaring gap for science. Scientists had to make educated guesses about its properties. Now, we can finally measure some of them."

The researchers employed a technique called chelation, where radioactive promethium is bound with special organic molecules. Using X-ray spectroscopy, they were able to determine the properties of this complex, including the length of the promethium's chemical bond with neighboring atoms. This groundbreaking discovery fills a significant gap in the periodic table.

The study involved producing promethium-147, a specific isotope with a half-life of 2.62 years. This allowed the researchers to acquire enough promethium at a high enough purity to examine its chemical behavior.

Significantly, they were the first to demonstrate a property called lanthanide contraction in a solution, encompassing the entire lanthanide series including promethium. Lanthanide contraction refers to the unexpected shrinking of elements with atomic numbers between 57 and 71. 

As their atomic numbers increase, the size of their ions shrinks. This unique phenomenon results in distinct chemical and electronic properties due to a shrinking space containing the same electrical charge.

The team's findings provided a clear signal for promethium, enabling them to better define the trend across the entire lanthanide series. "It's truly remarkable from a scientific standpoint," remarked Dr. Ivanov. "The contraction of this chemical bondchemical bond accelerates along the series, but slows down considerably after promethium."

This achievement represents a major leap in understanding the bonding properties and structural changes of these elements within the periodic table. "Among other benefits," explained Dr. Santa Jansone-Popova, another researcher involved in the study, "this will simplify the challenging task of separating these valuable elements. Our team has been working on separation techniques for the entire lanthanide series, but promethium was the missing piece. Separating them is a complex process, and the contraction plays a crucial role."

"Modern technological marvels rely on these rare earth elements in some way," Dr. Popovs concluded. "With this discovery, we've finally added the missing link to the puzzle."


D.M. Driscoll et al. Observation of a promethium complex in solution. Nature, published online May 22, 2024;  doi: 10.1038/s41586-024-07267-6