The manner of radioactive breakdown fits the theory, too. Suppose a nucleus gives off an alpha particle. The alpha particle is a helium nucleus made up of 2 protons and 2 neutrons. If a nucleus loses an alpha particle, its mass number should decline by 4 and its atomic number by 2, and that is what happens.
Suppose a nucleus gives off a beta particle. For a moment, that might seem puzzling. If the nucleus contains only protons and neutrons and no electrons, where does the beta particle come from? Suppose we consider the neutrons as proton-electron combinations. Within many nuclei, the neutrons are quite stable and do not break up as they do in isolation. In the case of certain nuclei, however, they do break up.
Thus the thorium-234 nucleus is made up of 90 protons and 144 neutrons. One of these neutrons might be viewed as breaking up to liberate an electron and leaving behind an unbound proton. If a beta particle leaves then, the number of neutrons decreases by one and the number of protons increases by one. The thorium-234 nucleus (90 protons, 144 neutrons) becomes a protactinium-234 nucleus (91 protons, 143 neutrons).
In short, the proton-neutron theory of nuclear structure could explain all the observed facts just as well as the proton-electron theory, and could explain the nuclear spins, which the proton-electron theory could not. What’s more, the isolated neutron had been discovered.
The proton-neutron theory was therefore accepted and remains accepted to this day.
The Nuclear Interaction
In one place, and only one, did the proton-neutron theory seem a little weaker than the proton-electron theory. The electrons in the nucleus were thought to act as a kind of glue holding together the protons.
But the electrons were gone. There were no negative charges at all inside the nucleus, only the positive charges of the proton, plus the uncharged neutron. As many as 83 positive charges were to be found (in the bismuth-209 nucleus) squeezed together and yet not breaking apart.
In the absence of electrons, what kept the protons clinging together?
Was it possible that the electrical repulsion between 2 protons is replaced by an attraction if those protons were pushed together closely enough? Can there be both an attraction and a repulsion, with the former the more important at very short range? If this were so, that hypothetical attraction would have to have two properties. First, it would have to be extremely strong—strong enough to overcome the repulsion of two positive charges at very close quarters. Secondly, it would have to be short-range, for no attractive force between protons of any kind was ever detected outside the nucleus.