How is the periodic table arranged by atomic number?

The periodic table is organized by arranging all chemical elements in order of increasing atomic number. The atomic number represents the total number of protons found within the atomic nucleus, which is also equal to the number of electrons in a neutral atom.12

This arrangement is fundamental to chemistry because it reveals the "periodic law," a recurring pattern where elements in the same vertical column, or group, exhibit similar chemical properties. These similarities arise because elements within the same group share comparable electronic configurations, particularly regarding their outermost valence electrons.13

While early versions of the table were based on atomic mass, the modern system was refined in the 1910s following the work of Henry Moseley, who used X-ray studies to confirm that the nuclear charge—the atomic number—is the defining characteristic that determines an element's place in the periodic system.14