jan 1, 1913 - Henry Moseley

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The periodic table was arranged by atomic mass, and this nearly always gives the same order as the atomic number. However, there were some exceptions (like iodine and tellurium), which didn’t work. Mendeleev had seen that they needed to be swapped around, but it was Moseley that finally determined why.

He fired the newly-developed X-ray gun at samples of the elements, and measured the wavelength of X-rays given. He used this to calculate the frequency and found that when the square root of this frequency was plotted against atomic number, the graph showed a perfect straight line. He’d found a way to actually measure atomic number.

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