Wrong position in the periodic table?

Lawrencium's ionization energy measured for the first time raises questions

Ionization energy of Lawrencium (red) and other elements in the periodic table © Kazuaki Tsukada / JAEA
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Controversial Heavyweight: The Lawrencium element may be in the wrong place in the periodic table. As an experiment now shows for the first time, its ionization energy is amazingly low - lower than that of all other actinides and lanthanides. This heats up the discussion about the position of this short-lived element - and about the order of the periodic system as a whole.

The position in the periodic table reflects the basic properties of an element: in which group they stand reveals, for example, how many electrons of their outer shell they give off in a chemical reaction or how many electrons they receive from the reaction partner. The ease with which this happens, can be read off the position. The large blocks of the periodic table show which electron orbital plays the decisive role.

A question of electrons

To find out where an element belongs in the periodic table, one must therefore know the arrangement of its electrons, but also its ionization energy - the energy needed to take away one of its external electrons. With everyday, stable elements such as oxygen or iron, this is not difficult with the super-heavy, sometimes extremely short-lived elements that frolic at the bottom of the system, but already.

The blocks of the periodic table. Lawrencium belongs to the f-block with the remaining actinides and lanthanides. © public domain

One of these shaky candidates is Lawrencium (Lr), because his ionization energy was unknown. For the time being, the element with the atomic number 103 is at the very end of the actinoids, a group that together with the lanthanides forms a separate block in the periodic table. In this f-block, the outer shell remains unchanged, but the atoms each receive an electron in the third outermost f orbital from element to element.

Only 27 seconds time

But new data may now question this position of Lawrencium. Tetsuya Sato of the Japanese Atomic Energy Agency JAEA and his colleagues has for the first time succeeded in ionizing Lawrencium and measuring the energy required and their results contradict the current classification according to some chemists. display

For their experiment, Sato and his colleagues first generated Lawrencium atoms by shooting boron atoms onto a Californium target an atomic number 98. Every few seconds, a Lawrencium isotope with a half-life is produced of 27 seconds. This gave the researchers just enough time to capture these atoms in a cloud of cadmium iodide and direct them to a metal plate around 2, 500 degrees Celsius. This heat catapulted an electron out of the orbitals on some of the atoms it was ionized.

Ionization potential of heavy lanthanides (black) and actinides (red) with the current result for Lawrencium (Lr). Tetsuya K. Sato / JAEA

Elektronklau was surprisingly easy

But when the researchers calculated the ionization energy, they had a surprise: it was only 4.96 electron volts. "That's unusually low, " Sato told the journal Nature. Because Lawrencium not only makes it easier to steal an electron than all the other lanthanides and actinides, it also has the fifth lowest ionization energy of the entire periodic table.

But what does this mean for the electron configuration of Lawrencium and for its position in the periodic table? There is no agreement on this so far. Some authors of the "Nature" paper still consider the element the best in the f-block. Christoph D llmann of the University of Mainz states: "We show that the energy required to dissipate the first electron at Lawrencium is, as expected, the lowest of all actinides." Take Lawrencium's position as the last member of the actinoid series and underpin the architecture of the Periodic Table of the Elements.

or wrongly positioned?

But there are also chemists who see it very differently, among them William Jensen from the University of Cincinnati. He argues that Lawrencium does not match the other actinides. Like lutetium, the last lanthanoid does not belong to his group. Because in both of them the ionization energy drops steeply compared to their neighbors. In addition, all other actinides have a higher ionization energy than the overlying lanthanides, only in Lawrencium and Lutatium it is vice versa. In his view, Lawrencium would instead stand directly below scandium and yttrium and thus in the d-block of the periodic table.

The International Union of Pure and Applied Chemistry (IUPAC), the world's leading panel on all official chemical issues, has not taken either position, as Jan Reedijk, President of the Inorganic Section of IUPAC, told Nature, However, the panel will discuss this issue during a summer session. (Nature, 2015; doi: 10.1038 / nature14342)

(Nature News / Nature / University Mainz, 09.04.2015 - NPO)