Moseley's law describes an approximately linear relation between the square root of a characteristic X-ray frequency and the atomic number: for a specified line series. The constants depend on which transitions are compared; represents an effective screening correction rather than a universal number for every line.
Henry Moseley bombarded samples with electrons and measured their characteristic X-ray wavelengths using a crystal spectrometer. Plotting the corresponding square-root frequencies revealed a systematic ordering by nuclear charge, including gaps where elements were missing. This helped distinguish the physical atomic number from ordering by atomic mass. Different line series and departures from exact straightness contain additional atomic-structure information; the simple relation is not an exact description of all X-ray transitions.[ref]
