Although the modern periodic table barely resembles the one constructed by Dmitri Mendeleev, every chemistry student learns that the placement of missing elements in the open slots of Mendeleev’s table was a scientific triumph. The discovery of isotopes in the early 1900s was an inflection point in periodicity, and chemistry as a discipline. Chemists once characterized each new isotope as a unique element—but as isotopes proliferated, fitting them into the existing periodic table became impossible. Several decades passed before the concept of isotopy fully developed. At that point, scientists seemingly concluded that chemistry occurred at the atomic level, and isotopic differences were the purview of physics. Had a different understanding of isotopy prevailed, the direction of chemistry could have changed dramatically. While the trajectory of synthetic chemistry might have remained constant, ‘chemists’ may have dominated the discovery of new superheavy elements by appropriating the modern conventional definition of ‘nuclear physicist’.