Remember the periodic table from your high school classroom? The table orders elements by how the valence electrons in their atoms are arranged. However, quantum physics calculations predict that in elements with atomic number greater than 103, the electrons are arranged in new, unusual ways.
This is because these electrons move so fast that their orbits are warped, as described by Einstein’s special theory of relativity — but verifying this has been tricky. These so-called superheavy elements don’t exist in nature and must be created one atom at a time in expensive nuclear reactions. The atoms also decay within seconds or even milliseconds. Thus, traditional chemical techniques such as liquid‑liquid extraction or gas chromatography can’t directly identify the exact molecule that short-lived atoms form.
This ambiguity has already produced contradictory results. For example, scientists have found that flerovium-114 sometimes acts like a noble gas and sometimes like a reactive metal.
In July, researchers with the Lawrence Berkeley National Laboratory in California reported that they had developed a new technique that overcame those hurdles by combining two existing instruments: a gas-filled separator and FIONA (“for the identification of nuclide A”). The gas-filled separator can sift superheavy atoms out of a blizzard of particles. FIONA is a sequence of traps, electrostatic lenses, and a cross‑field mass spectrometer that can cool ions to near‑room temperature, manipulate them, and finally measure each ion’s mass‑to‑charge ratio.
The findings were published in The Journal of Physical Chemistry A.
Because truly superheavy isotopes like rutherfordium and dubnium are too scarce to use to develop new methods, the researchers used radioactive holmium as a stand‑in. Holmium can be produced fast enough to collect good data yet is rare enough to mimic superheavy elements.
To make the isotopes, the researchers bombarded an indium target with a beam of ionised argon-40. The resulting fusion‑evaporation reactions yielded a few hundred 151Ho or 152Ho nuclei per second plus a plethora of other ions.
A low‑pressure helium atmosphere inside the separator quickly stripped or attached electrons so that most holmium ions reached charge states of +5 or +6. The ions were then exposed to a magnetic field, which deflected the holmium ions alone by a particular degree, and in this way they were steered towards FIONA. The uninteresting ions were made to hit absorber walls.
At FIONA, a gas catcher lowered the ions’ charged state to +1 and cooled them to room-temperature energies. Finally, they were confined by an ion trap within a millimetre‑scale volume, where they would have a lifetime of 50-100 milliseconds. This was long enough to perform chemical reactions before the superheavy atoms decayed.
At this stage, the researchers bled a stream of oxygen molecules (O2) into the trap. Each Ho⁺ ion could either stay bare or react to make HoO⁺.
After the chosen trap time, electric pulses forced the ions to exit the trap, where a sensitive spectrometer awaited. 151Ho and 152Ho release well‐known and distinct amounts of energy when they decay and have well-known and distinct half‑lives (35 and 162 seconds respectively). The spectrometer used these details to ‘sniff’ out the specific isotope in each molecule as the molecules decayed.
In this way, the researchers were able to identify radioactive molecules directly, one atom at a time, formed by the holmium isotopes, with no confusion from background or contaminant signals. The amount of HoO⁺ detected increased by predictable amounts in response to both longer exposure times and higher oxygen gas flow, confirming that the technique could track molecular formation rates under varying conditions.
Importantly, the measurements confirmed that most of the holmium ions could be converted to HoO⁺ within around 50 milliseconds at higher oxygen flow rates. They also reported finding no evidence for the formation of other, unexpected holmium oxygen compounds, confirming that the main reaction product was HoO⁺.
The findings may open the door to a direct chemistry of rare atoms. By tracking which fluorides, oxides or other ligated ions actually form and how quickly, researchers can use the gas-filled separator followed by FIONA to infer how readily the electrons in a superheavy atom access d‑ or p‑orbitals in their atoms. This information is directly tied to the element’s place in the periodic table.
For example, observing whether RfO2+ or DbO3+ forms under standard conditions can be used to check long‑standing predictions about how superheavy atoms in group 4 and group 5 of the table behave.
This is all the more relevant science scientists have also been developing new techniques to increase the production rate of superheavy elements. Usually, they have made these nuclei by bombarding a target element with a high-energy beam of atoms of another element.
The traditional choice of projectile beam has been calcium-48, but of late they have been switching to the more accessible 40Ar. For example, scientists have predicted the reaction of 40Ar with berkelium-249 to be an efficient way to synthesise important superheavy nuclei like moscovium-286. Scientists have also deployed different ion beams, such as titanium-50, to strike heavy targets like plutonium-244 to produce livermorium-116 in greater quantities. Experts have expressed belief that this method is a promising path to create even heavier elements, like element 120.
Innovations such as the development of highly-sensitive custom detector systems also allow scientists to rapidly and reliably detect single superheavy nuclei as soon as they are formed. Similarly, advances in target assembly, including multilayered targets designed to withstand more intense projectile beams, allow experiments to go on for longer and yield more superheavy nuclei. Scientists have also been using more refined models to fine-tune accelerator parameters, such as beam intensity and energy, which can improve the chances of specific nuclei forming over others.
As production rates inch upward and techniques like FIONA become more mature, chemists can start asking finer questions. For example, which oxidation states dominate in a molecule? How strong are the ligand bonds involving superheavy elements? And do their 7p– or 6d-orbital electrons participate in chemistry the way the theory predicts?
But before scientists can actually explore these questions, they will have to surmount some important challenges. This is because, as the researchers acknowledged in their paper, holmium is only an approximate stand-in. Actual superheavy elements will present unique challenges. For three examples:
First, real superheavy elements often have very short half-lives, sometimes much less than a second. This narrows the time available to produce, trap, and chemically analyse them before they decay. Second, and crucially, the technique was designed to detect only specific charge-to-mass species in each run, e.g. only HoO⁺ or Ho⁺ per measurement. As a result, unexpected molecular forms might go undetected.
Third: the study demonstrated only relatively simple chemical reactions. Superheavy elements may form more complex, less predictable molecules or may behave uniquely, and it isn’t yet clear whether the technique can fully resolve or identify a wide variety of more complex reaction products.