The number of protons in the nucleus of the atom
is, therefore, exactly Z, the atomic number Discovery of Neutron
Since the nuclei of deuterium and tritium are isotopes of hydrogen, they
must contain only one proton each But the masses of the nuclei of
hydrogen, deuterium and tritium are in the ratio of 1:2:3 Therefore, the
nuclei of deuterium and tritium must contain,  in addition to a proton,
some neutral matter