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Henry Moseley measured characteristic X-ray frequencies across a sequence of chemical elements. For a given series, the square root of the frequency is approximately linear in atomic number after a screening offset is included. The regularity connects a spectral measurement with a discrete nuclear-charge label, rather than merely correlating with atomic weight. Missing positions in that sequence also made X-ray spectroscopy a way to test and extend the periodic ordering of chemical elements.^[ref]

A physicist.^[ref]

Moseley's law describes an approximately linear relation between the square root of a characteristic X-ray frequency and the atomic number: $\sqrt{\nu }=a(Z-b)$ for a specified line series. The constants depend on which transitions are compared; $b$ 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]