The higher an element’s atomic number Z, the greater the effect of special relativity on its atomic structure. The innermost, speediest electrons are the most directly affected: Their relativistic mass causes their orbitals to contract, and the resulting change in nuclear charge screening alters the sizes and energies of all other electronic orbitals. The magnitude of the relativistic correction is small but noticeable in elements with Z as low as 30; it grows extremely rapidly in the heaviest elements, whose core electrons can exceed 75% of the speed of light.

The basic physics of the effect is well understood, but determining the consequences for large atoms poses a chicken-and-egg problem. Elements beyond fermium (Z = 100) can be made only one atom at a time. It’s possible to use laser spectroscopy to probe the energy levels of a single atom in a trap—but only if one already knows...

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