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How did atomic number replace atomic weight as the basis for organizing the periodic table?

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Periodic table of the elements with color-coded categories and labeled element groups, showing atomic numbers, symbols, and basic information for each chemical element. — periodic table Modern version of the periodic table of the elements. To learn an element's name, atomic number, electron configuration, atomic weight, and more, select one from the table.

Early versions of the periodic table, such as those developed by Dmitri Mendeleev, organized elements primarily by atomic weight. However, this method faced challenges because certain elements—such as the pairs argon/potassium, cobalt/nickel, and tellurium/iodine—had to be placed out of strict atomic weight order to align with their chemical properties.1

The transition to using atomic number as the organizing principle occurred in the early 20th century as scientists gained a deeper understanding of atomic structure. In 1911, Antonius van den Broek suggested that an element's position in the periodic table corresponded to its atomic number, which represents the number of positive charges in the atomic nucleus.1

This hypothesis was confirmed in 1913 by the physicist Henry Moseley. Through his experiments with X-ray spectra, Moseley demonstrated that the frequencies of characteristic spectral lines depend systematically on an element's atomic number. This discovery proved that atomic number, rather than atomic weight, is the fundamental property determining an element's identity and its correct placement within the periodic system.231