Nuclear Scientists Identify Nobelium Molecules, Redefining Chemistry

Nuclear scientists at the Lawrence Berkeley National Laboratory (LBNL) in the United States have made a groundbreaking advancement by producing and identifying molecules that contain nobelium for the first time. Nobelium, with an atomic number of 102, is the heaviest element ever observed in a directly identified molecule. This achievement, according to team leader Jennifer Pore, could significantly alter our understanding of the periodic table’s lower regions.

In a recent study published in Nature, Pore explained how the research team compared the chemical properties of nobelium with those of actinium (atomic number 89) to gain insights into the behavior of heavy elements. “The success of these measurements demonstrates the possibility to further improve our understanding of heavy and superheavy-element chemistry,” Pore stated, emphasizing the importance of correctly placing these elements on the periodic table.

The periodic table currently consists of 118 elements, arranged into vertical “groups” and horizontal “periods” based on their atomic numbers. Elements such as actinium and nobelium belong to the actinide series, often depicted as an offset block below the main table. This arrangement helps scientists intuitively grasp the chemical properties of various elements and allows predictions about newly discovered or laboratory-created elements.

As Pore elaborated, the traditional patterns of the periodic table may begin to falter when it comes to the heaviest elements. “These heavy nuclei contain a large number of protons, which creates intense electromagnetic forces that can influence electron behavior,” she noted. In the actinides, the strong charge from additional protons can lead to relativistic effects, potentially altering the expected chemical properties.

Researching elements heavier than fermium (Z = 100) presents unique challenges, as these elements must be produced and studied atom by atom. This necessitates the use of advanced equipment like accelerated ion beams and the FIONA (For the Identification Of Nuclide A) device at LBNL’s 88-Inch Cyclotron Facility.

The team specifically chose to study actinium and nobelium because they represent the extremes of the actinide series. Actinium, being the first element in the series, has no electrons in its 5f shell and is exceedingly rare. Recent research has only begun to reveal its crystal structure. In contrast, nobelium has a complete set of 14 electrons in its 5f shell, but its chemistry remains less understood.

To conduct their research, the team produced and identified molecular species containing actinium and nobelium ions. They generated these ions by directing beams of 48 Ca onto targets of 169 Tm and 208 Pb. Following this, they utilized the Berkeley Gas-filled Separator to isolate the resulting actinide ions from the unreacted material and by-products.

The next phase involved injecting the ions into a chamber of the FIONA spectrometer, filled with high-purity helium and trace amounts of H2O and N2. This chamber operated at a pressure of approximately 150 torr. Interactions with the helium gas reduced the actinide ions to their 2+ charge state, enabling the formation of “coordination compounds” with the impurities. This crucial step occurred either within the gas buffer cell or as the gas-ion mixture exited via a narrow opening, transitioning into a low-pressure environment. This transition induced rapid cooling, stabilizing the molecular species.

Once the actinide molecules formed, they were transferred to a radio-frequency quadrupole cooler-buncher ion trap. This trap confined the ions for up to 50 ms, allowing them to collide with the helium buffer gas and achieve thermal equilibrium. After cooling, the molecules were reaccelerated using FIONA’s mass spectrometer, where they were identified based on their mass-to-charge ratio. FIONA’s speed and sensitivity represent significant advancements in studying heavy and superheavy elements, which tend to decay quickly.

According to Pore, previous experiments primarily measured secondary particles generated from decaying molecules containing superheavy elements but failed to identify the original chemical species. “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,” she explained.

This research not only enhances our understanding of heavy and superheavy elements but may also have practical applications. For instance, the 225 Ac isotope shows potential for treating certain metastatic cancers but is challenging to produce and often available only in limited quantities. “If we could better understand these radioactive elements, we might facilitate the production of the specific molecules needed for treatment,” Pore remarked.

The implications of this research extend beyond theoretical chemistry, potentially impacting medical treatment and our foundational understanding of the elements that comprise the periodic table. The work conducted at LBNL represents a significant step forward in the ongoing exploration of the properties and behaviors of the universe’s heaviest elements.