Nuclear Scientists Create Molecules with Superheavy Nobelium

Nuclear scientists at the Lawrence Berkeley National Laboratory (LBNL) in the United States have achieved a significant milestone by producing and identifying molecules that contain nobelium for the first time. This element, with an atomic number of 102, is the heaviest element ever observed in a directly identified molecular form. Team leader Jennifer Pore emphasizes that this groundbreaking work could lead to a re-evaluation of the periodic table, particularly concerning the positioning of heavy and superheavy elements.

Pore, a research scientist at LBNL, explained, “We compared the chemical properties of nobelium side-by-side to simultaneously produced molecules containing actinium (element number 89).” The successful measurements of these molecules enhance the understanding of heavy and superheavy element chemistry and may ensure their accurate placement on the periodic table.

Understanding the Periodic Table’s Structure

The current periodic table features 118 elements, arranged in vertical groups and horizontal periods. Elements share similar properties within groups, while atomic numbers (the number of protons in the nucleus) increase from left to right across periods. The arrangement also includes three distinct blocks, one of which encompasses actinides like actinium and nobelium. This offset positioning aids scientists in grasping the chemical properties of various elements and enables predictions regarding newly discovered or synthesized elements.

Pore highlighted a critical issue: traditional patterns may begin to falter for the elements at the bottom of the table. This disruption could challenge the predictive nature of the periodic table as it currently stands. The underlying factor, according to Pore, is the substantial number of protons in these heavy nuclei. For instance, the actinides (Z > 88) possess an intense positive charge that significantly influences the behavior of inner electrons, leading to relativistic effects that can alter chemical properties.

“As some electrons are drawn towards the nucleus, they shield outer electrons from the nucleus’s pull,” Pore said. “This effect is expected to be even more pronounced in superheavy elements, potentially misplacing them on the periodic table.”

Innovative Techniques for Heavy Element Research

Researching the impacts of these relativistic effects poses challenges. Elements heavier than fermium (Z = 100) must be produced and studied individually, necessitating advanced equipment such as accelerated ion beams and the FIONA (For the Identification Of Nuclide A) device at LBNL’s 88-Inch Cyclotron Facility. The team opted to investigate actinium and nobelium as they represent the extremes of the actinide series, with actinium being the first in the series and nobelium being the heaviest.

To produce and identify molecular species containing actinium and nobelium ions, the researchers accelerated beams of 48 Ca onto targets of 169 Tm and 208 Pb, respectively. Following this, they utilized the Berkeley Gas-filled Separator to isolate the resulting actinide ions from unused beam material and reaction by-products. The ions were then introduced into a gas catcher chamber within the FIONA spectrometer, filled with high-purity helium and trace amounts of water and nitrogen at a pressure of approximately 150 torr.

This innovative process reduced the actinide ions to a 2+ charge state, enabling the formation of coordination compounds between the actinide ions and the impurities. The compound formation occurred in the gas buffer cell or as the gas-ion mixture transitioned to a low-pressure environment, allowing the molecular species to stabilize.

Once the actinide molecules were formed, the researchers transferred them to a radio-frequency quadrupole cooler-buncher ion trap. This ion trap confined the molecules for up to 50 ms, allowing them to collide with the helium buffer gas until they reached thermal equilibrium. The cooled molecules were then re-accelerated using FIONA’s mass spectrometer, where they were identified based on their mass-to-charge ratio.

According to Pore, FIONA’s speed and sensitivity are crucial for studying the chemistry of heavy and superheavy elements, which are notoriously difficult to produce and decay quickly. “Previous experiments measured the secondary particles generated when a molecule with a superheavy element decayed, but they couldn’t identify the exact original chemical species,” she stated. “Our new approach is the first to directly identify the molecules by measuring their masses, eliminating the need for assumptions based on better-known elements.”

In addition to advancing the understanding of heavy and superheavy elements, this research may have implications for the development of radioactive isotopes used in medical treatments. For instance, the 225 Ac isotope shows promise for treating certain metastatic cancers, yet its production is challenging and limited to small quantities. This scarcity restricts access for clinical trials and treatments.

Pore noted, “This limitation has forced researchers to forgo fundamental chemistry experiments to determine how to deliver it to patients. But if we could better understand these radioactive elements, we might find an easier route to produce the specific molecules needed.”

The findings from this research, detailed in the journal Nature, pave the way for future studies on heavy elements and their potential applications, marking a significant step forward in the field of nuclear chemistry.