Nuclear scientists at the Lawrence Berkeley National Laboratory (LBNL) in the US have produced and identified molecules containing nobelium for the first time. This element, which has an atomic number of 102, is the heaviest ever to be observed in a directly-identified molecule, and team leader Jennifer Pore says the knowledge gained from such work could lead to a shake-up at the bottom of the periodic table.
“We compared the chemical properties of nobelium side-by-side to simultaneously produced molecules containing actinium (element number 89),” says Pore, a research scientist at LBNL. “The success of these measurements demonstrates the possibility to further improve our understanding of heavy and superheavy-element chemistry and so ensure that these elements are placed correctly on the periodic table.”
The periodic table currently lists 118 elements. As well as vertical “groups” containing elements with similar properties and horizontal “periods” in which the number of protons (atomic number Z) in the nucleus increases from left to right, these elements are arranged in three blocks. The block that contains actinides such as actinium (Ac) and nobelium (No), as well as the slightly lighter lanthanide series, is often shown offset, below the bottom of the main table.
The end of a predictive periodic table?
Arranging the elements this way is helpful because it gives scientists an intuitive feel for the chemical properties of different elements. It has even made it possible to predict the properties of new elements as they are discovered in nature or, more recently, created in the laboratory.
The problem is that the traditional patterns we’ve come to know and love may start to break down for elements at the bottom of the table, putting an end to the predictive periodic table as we know it. The reason, Pore explains, is that these heavy nuclei have a very large number of protons. In the actinides (Z > 88), for example, the intense charge of these “extra” protons exerts such a strong pull on the inner electrons that relativistic effects come into play, potentially changing the elements’ chemical properties.
“As some of the electrons are sucked towards the centre of the atom, they shield some of the outer electrons from the pull,” Pore explains. “The effect is expected to be even stronger in the superheavy elements, and this is why they might potentially not be in the right place on the periodic table.”
Understanding the full impact of these relativistic effects is difficult because elements heavier than fermium (Z = 100) need to be produced and studied atom by atom. This means resorting to complex equipment such as accelerated ion beams and the FIONA (For the Identification Of Nuclide A) device at LBNL’s 88-Inch Cyclotron Facility.
Producing and directly identifying actinide molecules
The team chose to study Ac and No in part because they represent the extremes of the actinide series. As the first in the series, Ac has no electrons in its 5f shell and is so rare that the crystal structure of an actinium-containing molecule was only determined recently. The chemistry of No, which contains a full complement of 14 electrons in its 5f shell and is the heaviest of the actinides, is even less well known.
In the new work, which is described in Nature, Pore and colleagues produced and directly identified molecular species containing Ac and No ions. To do this, they first had to produce Ac and No. They achieved this by accelerating beams of 48Ca with the 88-Inch Cyclotron and directing them onto targets of 169Tm and 208Pb, respectively. They then used the Berkeley Gas-filled Separator to separate the resulting actinide ions from unreacted beam material and reaction by-products.
The next step was to inject the ions into a chamber in the FIONA spectrometer known as a gas catcher. This chamber was filled with high-purity helium, as well as trace amounts of H2O and N2, at a pressure of approximately 150 torr. After interactions with the helium gas reduced the actinide ions to their 2+ charge state, so-called “coordination compounds” were able to form between the 2+ actinide ions and the H2O and N2 impurities. This compound-formation step took place either in the gas buffer cell itself or as the gas-ion mixture exited the chamber via a 1.3-mm opening and entered a low-pressure (several torr) environment. This transition caused the gas to expand at supersonic speeds, cooling it rapidly and allowing the molecular species to stabilize.
Once the actinide molecules formed, the researchers transferred them to a radio-frequency quadrupole cooler-buncher ion trap. This trap confined the ions for up to 50 ms, during which time they continued to collide with the helium buffer gas, eventually reaching thermal equilibrium. After they had cooled, the molecules were reaccelerated using FIONA’s mass spectrometer and identified according to their mass-to-charge ratio.
A fast and sensitive instrument
FIONA is much faster than previous such instruments and more sensitive. Both properties are important when studying the chemistry of heavy and superheavy elements, which Pore notes are difficult to make, and which decay quickly. “Previous experiments measured the secondary particles made when a molecule with a superheavy element decayed, but they couldn’t identify the exact original chemical species,” she explains. “Most measurements reported a range of possible molecules and were based on assumptions from better-known elements. Our new approach is the first to directly identify the molecules by measuring their masses, removing the need for such assumptions.”
As well as improving our understanding of heavy and superheavy elements, Pore says the new work might also have applications in radioactive isotopes used in medical treatment. For example, the 225Ac isotope shows promise for treating certain metastatic cancers, but it is difficult to make and only available in small quantities, which limits access for clinical trials and treatment. “This means that researchers have had to forgo fundamental chemistry experiments to figure out how to get it into patients,” Pore notes. “But if we could understand such radioactive elements better, we might have an easier time producing the specific molecules needed.”
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