Scientists made the first measurement of the bonding distance of einsteinium, a synthetic, highly radioactive element that has rarely been studied because it is difficult to produce and decays quickly.
The findings revealed some basic but unexpected chemical properties of einsteinium, and could inform the nature of other hard-to-test elements and even be used to discover new elements. The results were published Wednesday in Nature.
Einsteinium is the 99th entry on the periodic table of elements and is located on the bottom row as one of the actinides, along with uranium, plutonium and 12 others. Researchers at Lawrence Berkeley National Laboratory discovered the element in 1952 in the remnants of the first hydrogen bomb, nicknamed “Ivy Mike,” but little is known about how einsteinium interacts with other elements.
The fleeting, silvery metal does not appear on Earth naturally and is produced once every other year at the Oak Ridge National Laboratory in Tennessee, where the researchers received less than 200 nanograms — less than a millionth of a gram — of einsteinium after a few years on a waitlist.
Led by Rebecca Abergel, the heavy element group leader at the same Berkeley lab and the study’s lead author, the team set out to contribute experimental data on einsteinium to the sparse literature.
“Our goal is to push the limits or really try to build up any kind of fundamental knowledge we can on the trans-plutonium elements, because we just need to continue understanding what's happening in this [actinide] series,” said Abergel, who is also a professor of nuclear engineering at the University of California, Berkeley.
To learn how einsteinium chemically interacts with other molecules, the scientists bonded the sample with the well-understood molecule 3,4,3-LI(1,2-HOPO), or simply HOPO. They measured the lengths of the bonds between the atoms, known as bonding distance, and found it was shorter than expected. The luminescence of the einsteinium-HOPO compound was also measured, revealing that einsteinium’s electrons behave in fundamentally different ways than those of the other actinides.
According to Abergel, these deviations from theoretical predictions may be because einsteinium’s electrons move at such fast speeds that they are affected by special relativity — which was discovered in part by the element’s namesake, Albert Einstein — causing them to behave differently.
Researching the fickle element was not without its challenges. Abergel and her colleagues also needed to change their technique for analysis after finding that their sample was contaminated with californium, another heavy and radioactive element.
Further testing was halted in March in response to the COVID-19 pandemic. Also, because the team's isotope of einsteinium has a half-life of 276 days and radioactively decays by about 7% each month, most of it has since disappeared. Although what remains is too scant to undergo more chemical tests, Abergel said the colleagues still plan to study einsteinium’s radioactivity and that future studies are needed to flesh out its chemical properties.
“We were lucky to be able to get all of this experimental work done before COVID,” she said.
Although the testing was limited, results such as einsteinium’s bond distance can improve the understanding of patterns across the periodic table and inform the search for new elements beyond oganesson, the largest known element and number 118.
“The more we learn about each element, the more we learn about the whole series, and then we'll be able to establish trends and maybe develop some new theoretical concepts,” Abergel said. “Maybe we're after understanding trends in the whole periodic table, because that could help us understand new properties or features of the earlier actinides but also, potentially, we could start predicting what happens beyond.”
The article, “Structural and spectroscopic characterization of an einsteinium complex,” was published Feb. 3 in Nature. The authors of the study were Korey Carter, Kurt Smith, Leticia Arnedo-Sanchez, Tracy Mattox, Liane Moreau and Corwin Booth, Lawrence Berkeley National Laboratory; Katherine Shield and Rebecca Abergel, Lawrence Berkeley National Laboratory and University of California, Berkeley; Zachary Jones, Los Alamos National Laboratory; and Jennifer Wacker, Los Alamos National Laboratory and Georgetown University. The lead author was Rebecca Abergel.