# How do people get the proton number for each element from experiment?

How did people determine the proton number for each element from experiment in each decade of 20th century?

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Moseley, the physicist who 'fixed' the Periodic Table at the start of the 20th Century, did it by measuring X-ray spectra. The energy of the $K_\alpha$ X-ray emission line is proportional to $(Z^2 - 1)$, where $Z$ is the atomic number.