The order of the elements
Mendeleev left gaps for elements not yet found and predicted their properties; gallium, scandium and germanium filled them within 17 years. Moseley's X-ray measurements showed the order was nuclear charge, not weight. Quantum mechanics later explained why the pattern exists.
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Closed 1869, Dmitri Mendeleev; 1913, Henry Moseley ties order to atomic number.
Sources
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