Although the periodic table currently contains 118 different elements, many properties of the heaviest elements remain unknown. Over centuries, chemists have studied relatively common elements such as carbon, gold, lithium, or copper by performing experiments to determine each element’s properties: their melting point, solubility, and electronegativity. How different elements behave is reflected in the structure of the periodic table; elements found in the same column often exhibit similar chemical characteristics.
The heavy elements found near the end of the periodic table, however, are more difficult to analyze. Heavy elements are highly radioactive, and, as they increase in mass, they must be produced under such precise laboratory conditions that only a handful of scientific institutions have successfully documented them. Lawrence Livermore scientists are devising new experimental methods to measure these elements’ properties, despite their elusiveness, to advance fundamental science understanding and improve theoretical models of heavy-element interactions.
Gaps in the Data
Determining the properties of heavy elements (those heavier than uranium, the last naturally occurring element) is necessary for nuclear science applications ranging from nuclear forensics to energy generation and medical isotope production. “The very existence of the National Nuclear Security Administration and its associated laboratories, including Lawrence Livermore, comes from understanding this section of the periodic table,” says staff scientist Gauthier Deblonde. Livermore has been at the center of heavy-element research since its earliest days of analyzing postdetonation materials, an effort that requires detailed knowledge of the chemistry of heavy elements, as well as their fission, and their isotopes. For example, from the 1980s until 2022, Livermore collaborated with the Joint Institute for Nuclear Research in Dubna, Russia, as part of a multinational effort to test the limits of nuclear theories by producing larger and larger atomic nuclei. Livermore’s contributions were recognized by the International Union of Pure and Applied Chemistry through the naming of element 116, livermorium.
The properties of these elements must be observed through experiments because their behavior is too complex to be fully explained by chemical theory alone. For instance, theoretical models for hydrogen, the lightest element, are well established because the interaction between its single proton and electron can be described with exact mathematical solutions. This description forms the foundation of density functional theory (DFT), which scientists use to predict the bonding, stability, and reactivity of multiple atoms in a system. “But the simplicity ends there,” says Deblonde. “Heavy elements each contain roughly 100 electrons, 100 protons, and 150 neutrons. We quickly reach the limits of theoretical models and computation capabilities when attempting to predict how heavy elements operate.” For these massive atoms, complex interactions between the nucleus and orbiting electrons are impossible to model explicitly. Neither theory nor computation can keep track of how the forces from one subatomic particle almost instantaneously affect every other particle making up the atom. Moreover, these elements’ stability and bonding behavior further deviate from expectation when accounting for the special relativistic effects of outer shell electrons moving nearly at the speed of light.
The modeling task is a mathematical nonstarter that demands simplification. “We apply approximations to quantum mechanical models such as DFT to predict how an atom will behave in a compound, but the difficulty lies in knowing which approximation is best suited for the given situation, such as whether the element is in isolation or interacting with inorganic compounds. The usual protocol is to compare our predictions with properties found in chemical databases, but hardly any data exists for the heaviest elements,” says researcher ShinYoung Kang. By making empirical measurements of heavy-element properties, researchers improve and add to the chemical data necessary to predict how these elements interact and decay. The main challenge is the extremely limited materials available to them.
Crystal-clear Views of Actinides
Continuing Livermore’s legacy of heavy-element investigation, Deblonde leads a research team describing new experimental techniques to measure the properties of actinides (elements 89 to 103 on the periodic table) using extremely small amounts of testable material. The elements in this group behave unpredictably in part due to their electronic complexity and their ability to form bonds in multiple oxidation states. Some actinides bear similarity to transition metals, which are generally found in the center of the periodic table, while others resemble the lanthanide elements in the row directly above them. Many actinide properties remain unmeasured because of the elements’ production cost, experimental difficulty, and the stringent safety protocols associated with handling radioactive materials. Standard laboratory assays for measuring chemical properties usually require milligram-size samples, yet the annual global production of certain actinides falls short of this amount by orders of magnitude. New techniques are necessary to make actinide experimentation practicable while adhering to material controls.
“Rather than demand more material to work with, we use a new class of molecules that allows us to study the properties of actinides while using only trace amounts of material,” says Deblonde. His strategy is to insert actinides into dense molecules. Using only microgram-size samples (1 microgram is 1,000 times smaller than 1 milligram) of an actinide source, this process creates crystals whose properties the team analyzes through a suite of chemistry techniques including crystallography, microscopy, fluorescence, and magnetic resonance spectroscopy. Begun as a Laboratory Directed Research and Development (LDRD) project in 2019, Deblonde’s team has since used this technique to produce several compounds with transuranic elements (elements heavier than uranium) and enable the measurement of these elements’ properties.
The process begins with a class of molecules called polyoxometalates (POMs), which consist of several oxygen atoms and metal atoms (often tungsten) huddled into connected pyramid- and trapezoid-shaped configurations. “POMs are a well-researched area of chemistry, but surprisingly, we found little existing literature applying them to actinide chemistry,” says Ian Colliard, a postdoctoral researcher on Deblonde’s team. Researchers cleave off a portion of the POM and replace the missing piece with the actinide elements to be studied. Although other atoms in the POM complex constitute the majority of the structure’s overall mass, the inserted actinide causes changes in the new complex’s geometry and spectroscopic properties in ways that divulge the actinide’s chemical properties. Other challenges are reduced as well by using this technique. Researchers preserve the national resource of radioisotopes and the program funding that would otherwise go to purchasing large, testable amounts of actinides, while also minimizing the safety risks associated with testing sizable sources of radioactive material.
Deblonde’s team is the first to have synthesized a number of transuranic element-bearing POM complexes. In doing so, the researchers have already encountered discrepancies between predictions of these elements’ properties and their empirical measurements. Actinides heavier than uranium are often thought to exhibit the same chemical behaviors as the lanthanide-series element directly above them on the periodic table, known as an element’s homolog. While this notion is valid for simple compounds, the team’s experiments with neptunium, plutonium, americium, curium, and californium yielded compounds significantly different than if they contained their homologs. For example, the team formed a wider variety of compounds from curium than from its lanthanide match, gadolinium. Plutonium also yields different crystal structures relative to its nonradioactive homologs. “Our previous chemical intuition would not have led us to predict this behavior. We still don’t know much about these heavy elements, including the basic crystal structures they will form,” says Deblonde, adding, “Our team is privileged to do this type of work. When measuring an actinide property or synthesizing a new compound, every action pushes the frontier of scientific knowledge. On any day at work, we could be the first in all of history to make a certain measurement, which is truly exciting.”
One Atom at a Time
Some elements are so challenging to produce and so unstable that they cannot be assessed through standard techniques. Superheavy elements (also called transactinides) are produced one atom at a time in a particle accelerator, and they often decay within seconds of their creation. As with the actinides, the size and complexity of transactinides makes accurately predicting their chemical behavior challenging for scientists. Electrons orbiting the nucleus of superheavy elements move at relativistic speeds, causing contraction of the innermost electron shells while the furthest-reaching shells expand. These changes affect how an atom forms chemical bonds with neighboring atoms, meaning superheavy elements may not behave as their placement on the periodic table (determined by the number of protons) would suggest. For instance, scientists theorize that the element flerovium could behave similarly to its homolog, lead, or instead like a noble gas such as radon. The only way to know for sure is for researchers to characterize these elements through laboratory experiments, whose many pieces of equipment must be timed and tuned to make use of precious, fleeting resources.
Automated Analysis
The brief half-lives of superheavy elements prevent scientists from handling the materials or intervening in an experimental process. An automated system developed by radiochemist John Despotopulos and his team extracts superheavy elements and then verifies their presence through the following steps:
- An accelerator—in this case, the 88-inch cyclotron at Lawrence Berkeley National Laboratory—produces superheavy elements (flerovium or copernicium).
- The superheavy elements achieve near-thermal equilibrium inside a gas recoil chamber.
- The elements are transported with the help of added water aerosol particles and a stream of helium carrier gas.
- A centrifuge separates the heavy element-bearing aerosols from the helium gas.
- An added solution forms a liquid with dissolved transactinide atoms.
- The liquid interacts with thiacrown ether molecules within an automated microfluidic system to chemically extract the element of interest.
“For superheavy elements with half-lives on the order of 30 seconds or less, the process from initial element production to detection and measurement needs to be automated as much as possible. Experiencing any procedural or mechanical disconnects along the way could cause researchers to miss out on significant amounts of effort,” says radiochemist John Despotopulos, who leads a Livermore effort designing automated experimental methods for measuring transactinide properties. (See "Automated Analysis," right.) The effort, which also began as an LDRD project, will probe whether the transactinides copernicium and flerovium behave similarly to their group 12 and 14 homologs, mercury and lead, respectively. His team’s approach is to leverage the well-documented chemistry of mercury and lead, sending atoms of each through a series of chemical extraction steps and verifying their presence at the end with a detector. Then, the process is repeated using the corresponding transactinides seconds after they are produced in a particle accelerator. If the team successfully detects the heavy element using the same extraction steps as with its homolog, the finding would support the notion that the superheavy element has similar chemical properties to its homolog. However, if not detected, the objective of the experiment shifts to pinpointing at which stage the extraction sequence fails for the heavy elements and determining what procedural adjustments are necessary to ensure detection. The nature of the adjustments reveals how the superheavy element differs from its homolog.
The team provides experimental results in real time to collaborators at the University of Chicago, who use the data to refine theoretical chemistry models and eventually understand the behavior of superheavy elements, particularly copernicium. These simulations helped to identify thiacrown ethers as promising extraction molecules for mercury atoms, and work is ongoing, both through simulation and experimentation, to determine how well their behavior translates to superheavy-element extraction.
Even if researchers uncover surprising tendencies of actinides and superheavy elements, scientific consensus is anything but fast-moving. “An enormous amount of scrutiny would be required to reorganize any part of the periodic table,” says Despotopulos. However, as scientists gain a clearer picture of the most inaccessible elements, their work raises further, fascinating questions of what new information—or new elements altogether—are still to come.
—Elliot Jaffe
For further information contact Gauthier Deblonde (925) 423-2068 (deblonde1 [at] llnl.gov (deblonde1[at]llnl[dot]gov)).
