About the close proximity of neutrons and protons
After the discovery of protons, it has been suggested that they make up atomic nuclei. However, this assumption proved untenable, since the ratio of a nuclear charge to its mass does not remain constant for different nuclei, as it would be if there were only protons in a nucleus. For heavier nuclei, this ratio is lower than for the light ones, i.e., when transiting to heavier nuclei, the mass of a nucleus is growing faster than its charge. In 1920, Rutherford conjectured the existence of rigidly connected compact proton-electron pairs inside nuclei, electrically neutral objects — particles with the mass approximately equal to the mass of a proton. According to the proton-electron theory, the intrinsic angular momentum (spin) of a nitrogen nucleus was supposed to be a half-integer, because it must contain an odd number of fermions — 14 protons and 7 electrons. But in 1930, it became clear that the spin of nitrogen nuclei is an integer. This fact is called the nitrogen catastrophe. On the basis of this and other evidences, Heisenberg suggested that nucleus consists of protons and neutrons.