Henry Moseley discovered that the X-ray spectra of elements show a linear relationship between frequency and atomic number.

• He concluded that the Atomic Number ($Z$) (the number of protons) is the fundamental property of an element, not atomic mass.
• Modern Periodic Law: "The physical and chemical properties of the elements are periodic functions of their atomic numbers."
• Mendeleev used atomic mass, Newlands used Octaves, and Dobereiner used Triads.