When was atomic number shown to determine the order of the periodic table?

In 1913, Henry Moseley used X-ray spectra to show that each element has a unique nuclear charge, later called atomic number. His work explained why some elements had to be reordered in the periodic table even when their atomic weights suggested otherwise. This was a turning point in chemistry because it gave the table a physical basis rather than relying only on observed masses and properties. Moseley’s findings also helped predict missing elements with much greater confidence.