• When Mendeleev put his periodic table together, nobody knew about the existence of the nucleus.
  • In 1913, English physicist Henry Moseley examined x-ray spectra of a number of chemical elements.
  • He would shoot X-rays through crystals of the element and study the wavelengths of the radiation he detected.
  • His results led to the definition of atomic number as the number of protons contained in the nucleus of each atom.
  • He then realized that the elements of the periodic table should be arranged in order of increasing atomic number rather than increasing atomic mass.
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