Henry Moseley showed that atomic number, rather than atomic mass, determines the periodic-table order.
In 1913 and 1914, Moseley used X-ray spectroscopy to study the characteristic X-rays emitted by different elements. He found a systematic relationship between an element’s X-ray frequency and its atomic number, which corresponds to the number of protons in its nucleus.
This work clarified several puzzles in earlier periodic tables. Ordering elements by atomic mass could produce apparent irregularities, but ordering them by atomic number placed chemically similar elements in the correct sequence. Moseley’s result also provided a powerful way to identify gaps for elements that had not yet been discovered.
Dmitri Mendeleev had created the influential earlier table and successfully predicted undiscovered elements, but he did not have the modern nuclear definition of atomic number. Moseley’s research supplied the physical basis for the table’s ordering and helped establish atomic number as a fundamental property.