<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>
who discovered that all atoms of an element have the same number of protons
James Chadwick