From Physics Today: “Einsteinium chemistry captured”

Physics Today bloc

From Physics Today

18 Feb 2021
Johanna L. Miller

The creation of a rare molecule offers a glimpse of how atoms behave at the Periodic Table’s outer reaches.

To date, researchers have created more than two dozen synthetic chemical elements that don’t exist naturally on Earth. Neptunium (atomic number Z = 93) and plutonium (Z = 94), the first two artificial elements after naturally occurring uranium, are produced in nuclear reactors by the thousands of kilograms. But the accessibility of transuranic elements drops quickly with Z: Einsteinium (Z = 99) can be made only in microgram quantities in specialized laboratories, fermium (Z = 100) is produced by the picogram and has never been purified, and all elements after that are made just one atom at a time.

There are ways to probe the atomic properties of elements produced atom by atom (see, for example, Physics Today, June 2015, page 14). But when it comes to the traditional way of investigating how atoms behave—mixing them with other substances in solution to form chemical compounds—Es is effectively the end of the periodic table.

Now Rebecca Abergel (head of Lawrence Berkeley National Laboratory’s heavy element chemistry program) and her colleagues have performed the most complicated and informative Es chemistry experiment to date. They chose to react Einsteinium [Es] with a so-called octadentate ligand—a single organic molecule, held together by the backbone shown in blue, that wraps around a central metal atom and binds to it from all sides—to create the molecular structure shown in the figure. In their previous work, Abergel and colleagues used the same ligand to study transition metals, lanthanides, and lighter actinides. When they were fortunate enough to acquire a few hundred nanograms of Es from Oak Ridge National Laboratory, they used it on that as well.

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Credit: Adapted from K. P. Carter et al., Nature 590, 85 (2021)

Among other useful properties, the ligand acts as an antenna: It absorbs light in the UV and efficiently channels the energy to the central metal atom, which emits light at a range of longer wavelengths. That luminescence spectrum, which can be measured with just a tiny quantity of material, carries information about the central atom’s electronic energy levels.

Between the luminescence spectroscopy and complementary x-ray absorption measurements, the researchers discovered that Es differs significantly in its behavior from both its upstairs neighbor holmium and the lighter actinides. The difference is almost certainly due to relativistic effects. The more highly charged an atomic nucleus, the faster the electrons whiz around it. When the electron speed is a significant fraction of the speed of light, it affects the atom’s quantum states in a way that’s extraordinarily difficult to model.

All actinides exhibit relativistic effects, but the heavier ones especially so. Although Es is so scarce that its chemistry is unlikely to be of any technological importance, it could provide a benchmark for better theoretical understanding of the more abundant lighter actinides’ chemical behavior. Abergel and colleagues are especially interested in how those radioactive elements behave inside the human body—with an eye toward both harnessing their radiation as a cancer treatment and designing new drugs to treat radiation poisoning. (K. P. Carter et al., Nature 590, 85, 2021 [above]).

See the full article here .

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