Moseley and Atomic Number (1913)
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In 1913, Henry Moseley measured X-ray spectra from many elements and showed X-ray frequency rose with atomic number.
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Moseley's data let him order elements by atomic number directly, correcting places where atomic-weight ordering had elements out of sequence.
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Moseley's result confirmed atomic number, not atomic weight, as the table's true ordering principle — distinct from any line spectrum.