What property of the atom did Moseley associate with the wavelength of X rays emitted from an element in his experiments?

The unique integer assigned to each element, known as the atomic number. Most astonishingly, there is an unanticipated relationship between the frequency and the relative mass of each atom. Let’s rank the elements by atomic mass, and allocate an integer to each rendering to its ranking in order by mass. In the Periodic Table, this rank order number also agrees to the element’s position in the Periodic Table.