How did Henry Moseley’s X-ray research confirm the importance of atomic number?

Henry Moseley’s X-ray research provided the experimental evidence needed to confirm that an element's position in the periodic table is determined by its atomic number—the number of positive charges in the nucleus—rather than its atomic weight.123

By measuring the X-ray spectra of various elements, Moseley discovered that the wavelengths and frequencies of these rays follow a regular pattern based on the nuclear charge. This relationship, known as Moseley’s law, allowed him to order elements precisely from aluminum to gold.132

This discovery resolved inconsistencies in Dmitri Mendeleev’s original table, where certain elements—such as cobalt and nickel—had to be placed out of atomic weight order to align with their chemical properties. Moseley’s work proved that atomic number is the fundamental property defining an element's identity, allowing scientists to confirm the completeness of the periodic table and predict the existence of undiscovered elements.132