Chemistry
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Uranium is ubiquitous, which means both Fr and At are present in both earth and other celestial bodies. While the chemistries of the pair are wildly different, both are decay products of U, so can be expected to form in proximity to each other. Once in a while, atoms of the two meet and fall in love. Francium astatide (FrAt) does exist at parts-per-planet concentrations.

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