Rarest element potential

C&I Issue 7, 2013

Nuclear physicists have measured an important property of astatine – the rarest element on Earth – for the first time.

This research to determine the ionisation potential of astatine, by scientists at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland, could help chemists to develop applications for astatine in radiopharmaceuticals for cancer treatment, as well as developing theories that predict the structure of super-heavy elements.

Astatine, atomic number 85, was the last element present in nature with an unknown ionisation potential – the energy required to remove one electron from the atom, according to CERN. Discovered in 1940, it exists only in trace amounts in nature.

In 1964, scientists observed two spectral lines in the UV region from a sample of artificially produced radioactive isotopes. No other data on the atomic spectrum of astatine were known before now.

Physicists at CERN made artificial isotopes of astatine by bombarding uranium targets with high-energy protons. ‘None of the many short-lived isotopes used in medicine exist in nature; they have to be artificially produced by nuclear reactions,’ explains Bruce Marsh from CERN and the University of Manchester, UK. ‘What is different about astatine is that its scarcity in nature makes it difficult to study by experiment, which is why this measurement of one of the fundamental properties is a significant achievement.’

By using a series of precisely wavelength-tuned lasers, an international team of scientists found the ionisation potential to be 9.31751eV (S. Rothe et al, Nature, 2013, doi:10.1038/ncomms2819).

This information will help scientists understand more about astatine’s chemical reactivity. ‘The experimental value for astatine also serves for benchmarking theories that predict the atomic and chemical properties of super-heavy elements,’ says Andrei Andreyev from the University of York, UK, ‘in particular a recently discovered element 117, which shares very similar characteristics to astatine.’

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