Neutron stardust and the elements of Earth

Brett F. Thornton & Shawn C. Burdette

At its inception, the periodic table sorted elements by weight, so it may be surprising that the heaviest natural element on Earth remains controversial, or at best, nebulous. In the strange, perhaps-unfinished search for this weightiest nucleus, the only definitive conclusion is that it lies somewhere beyond uranium.

Credit: Chelsea Anne Barr

When asked “what is the lightest chemical element?” chemists can answer “hydrogen” with certitude. Even the follow-up “what is the lightest chemical isotope?” can be answered “hydrogen-1” with confidence. These answers are unambiguous because we know that 1H contains one proton and one electron, both subatomic particles that cannot be deconstructed to form a lighter chemical element. We are discounting exotic species such as positronium and muonium due to a lack of protons — and thus a lack of a position on the periodic table. Naming the lightest element was not always so easy: in the late nineteenth century scientists speculated about the existence of sub-hydrogen elements and even Mendeleev proposed them as a chemical explanation for the luminiferous æther1.

If you ask “what is the heaviest chemical element?” however, you may get a variety of answers along with a heap of uncertainty. Until the 1940s, the heaviest known element was uranium, and generations of students were taught this in general chemistry classes. But in science, there’s always someone ready to challenge conventional wisdom — so there must be something heavier than uranium, right? Whether or not it is truly possible to know the identity of the heaviest chemical element remains debatable as the periodic table expands. For our discussions here, by ‘heaviest element’ we mean the chemical element with the highest atomic number — the one with the most protons in its nucleus. While an isotope of a lighter element might have a higher absolute mass than a heavier known element, here we are interested in the atomic number since that accounts for (almost2) all of each element’s chemical uniqueness.

In 2015, IUPAC arbitrated the identity of the heaviest known chemical element by approving3,4 the claims for the artificial production of elements 113, 115, 117 and 118. This means that element 118, oganesson, currently holds the heaviest element distinction, but it’s a foregone conclusion that the periodic table has not reached its ultimate limit. Experiments are being designed to forge heavier elements5,6 and though many have speculated about how far the periodic table may extend7,8,9,10, we have only hypotheses for now. Since we do not know the ultimate limit to artificially produced elements, here we are seeking to answer the question: “what is the heaviest chemical element present naturally on Earth — an element not created by humans?”

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