First ionization energy of lawrencium determined

Measurements confirm position of lawrencium as final member of the actinide series and corroborate architecture of the periodic table


Thanks to a novel-type technique, a joint international research team has recently been able to measure lawrencium's first ionization energy. Lawrencium – element 103 – is a radioactive synthetic element that is not found in nature. It is the heaviest actinide, a group that consists of 15 elements with similar properties and that includes uranium and plutonium. Under the lead of researchers from the Japan Atomic Energy Agency (JAEA) in Tokai, the researchers succeeded in measuring lawrencium's first ionization energy and determined it to 4.96 electron volts. The first ionization energy is the minimum amount of energy needed to remove the most loosely bound electron from a neutral atom. "We found that the energy required to remove the outermost electron in lawrencium was the lowest among all the actinides," explained Professor Christoph Düllmann of Johannes Gutenberg University Mainz (JGU). The results confirm the position of lawrencium as the last actinide and also substantiate the current architecture of the periodic table of elements.

The chemical properties of an element are primarily determined by the electron configuration in the outermost occupied shell. Effects associated with the theory of Special Relativity have a considerable influence on electron structure in elements at the end of the periodic table, and this often has a direct impact on their chemical properties. One of the primary objectives of the analysis of the chemical and atomic properties of these elements is to determine the extent of this influence.

Following the introduction of the 'actinide concept' by Glenn T. Seaborg in the 1940s, which was the most dramatic revision of the periodic table of elements in recent history, lawrencium has played an extremely important role as the final member of the group of actinides. Due to its unique position, lawrencium has been in the focus of numerous studies to both determine the influence of relativity-related effects and uncover the properties that confirm that it is, in fact, the final element in the actinide family. Lawrencium was thus expected to have a very low ionization energy, much alike lutetium, the final element of the lanthanide group. However, it is no easy matter to measure the ionization energy of this element.

Lawrencium can only be produced in quantities of single atoms in heavy ion accelerators, and only isotopes with short lifetimes are known. Thus only very few experimental studies have been performed and have so far focused on selected chemical properties only. For their experiment using the JAEA tandem accelerator, the nuclear chemists employed a novel combination and advancement of methods and techniques that made measurement of lawrencium's ionization energy possible.

For the experiment, the Institute of Nuclear Chemistry at Mainz University purified and prepared the exotic target material californium (element 98). The material was converted into a target in Japan and then exposed to a beam of boron ions (element 5). The experiment was supplemented by theoretical calculations undertaken by scientists at the Helmholtz Institute Mainz (HIM) and at Tel Aviv University of Israel using the most up-to-date quantum chemical methods to quantify the ionization energy. The very good agreement between calculated and experimental result validates the quantum chemical calculations.

"Our experimental technique opens up new perspectives for similar examinations of even more exotic, superheavy elements," said the scientists expressing their expectations for the future. The international team involved research groups from JAEA, the Institute of Nuclear Chemistry at Mainz University, the Helmholtz Institute Mainz (HIM), the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, CERN in Geneva, Switzerland, the Japanese universities of Ibaraki, Niigata, and Hiroshima, Massey University in Auckland, New Zealand, and Tel Aviv University, Israel. These new findings have just been published in the scientific journal Nature.

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T. K. Sato et al., Measurement of the first ionization potential of lawrencium, element 103, Nature 520, 209-211, 9 April 2015,
View image in original size
View image in original size ill.: Kazuaki Tsukada, JAEA
The periodic table of elements including in the colored block at the bottom the lanthanides (Ln) and actinides (An). The height of each column indicates the relative first ionization potential of the corresponding element. The result obtained for lawrencium (Lr) is shown by the red column. The binding energy of the least bound valence electron in lawrencium is thus weaker than that in all other actinides and all other lanthanides.
View image in original size
View image in original size ill.: Tetsuya K. Sato, JAEA
The ionization energy of heavy lanthanides (black) and actinides (red) with the current result for lawrencium (Lr). The filled circle symbols represent values recorded in experiments, the ring symbols are estimated values. The two values for lawrencium are in excellent agreement, emphasizing the close correspondence between the theoretical expectations and experimental findings.
View image in original size
View image in original size photo: Tetsuya K. Sato, JAEA
The gray tantalum tube surrounded by two heating elements in the center of the picture is part of the recently developed surface ion source installed in the JAEA-ISOL system in the JAEA tandem accelerator.

Contact Contact
Professor Dr. Christoph Düllmann
Institute of Nuclear Chemistry
Johannes Gutenberg University Mainz
55099 Mainz
Tel +49 6131 39-25852
Fax +49 6131 39-20811

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