<P> In 1915 the reason for nuclear charge being quantized in units of Z, which were now recognized to be the same as the element number, was not understood . An old idea called Prout's hypothesis had postulated that the elements were all made of residues (or "protyles") of the lightest element hydrogen, which in the Bohr - Rutherford model had a single electron and a nuclear charge of one . However, as early as 1907 Rutherford and Thomas Royds had shown that alpha particles, which had a charge of + 2, were the nuclei of helium atoms, which had a mass four times that of hydrogen, not two times . If Prout's hypothesis were true, something had to be neutralizing some of the charge of the hydrogen nuclei present in the nuclei of heavier atoms . </P> <P> In 1917 Rutherford succeeded in generating hydrogen nuclei from a nuclear reaction between alpha particles and nitrogen gas, and believed he had proven Prout's law . He called the new heavy nuclear particles protons in 1920 (alternate names being proutons and protyles). It had been immediately apparent from the work of Moseley that the nuclei of heavy atoms have more than twice as much mass as would be expected from their being made of hydrogen nuclei, and thus there was required a hypothesis for the neutralization of the extra protons presumed present in all heavy nuclei . A helium nucleus was presumed to be composed of four protons plus two "nuclear electrons" (electrons bound inside the nucleus) to cancel two of the charges . At the other end of the periodic table, a nucleus of gold with a mass 197 times that of hydrogen, was thought to contain 118 nuclear electrons in the nucleus to give it a residual charge of + 79, consistent with its atomic number . </P> <P> All consideration of nuclear electrons ended with James Chadwick's discovery of the neutron in 1932 . An atom of gold now was seen as containing 118 neutrons rather than 118 nuclear electrons, and its positive charge now was realized to come entirely from a content of 79 protons . After 1932, therefore, an element's atomic number Z was also realized to be identical to the proton number of its nuclei . </P> <P> The conventional symbol Z possibly comes from the German word Atomzahl (atomic number). However, prior to 1915, the word Zahl (simply number) was used for an element's assigned number in the periodic table . </P>

Who discovered that the number of protons for each element’s atoms is unique