How did the discovery of atomic number improve the periodic table?

The discovery of the atomic number—the number of protons in an atomic nucleus—fundamentally improved the periodic table by providing a precise, scientific basis for the arrangement of elements.12

Previously, Dmitri Mendeleev and others organized elements primarily by atomic weight. This approach was inconsistent, as some elements, such as cobalt and nickel, had to be placed out of order to ensure their chemical properties aligned with their respective groups.413

In 1913, Henry Moseley confirmed through X-ray spectroscopy that an element's properties and its position in the periodic system are determined by its atomic number rather than its atomic mass. This shift resolved long-standing discrepancies in the table's order. Furthermore, because each atomic number corresponds to exactly one element, scientists could finally confirm the completeness of the periodic table, identifying exactly where new elements remained to be discovered. This transition also clarified that isotopes—atoms with the same atomic number but different weights—rightfully occupy the same position in the system.13