By the new theory, can we suppose that it is neutrons rather than electrons that somehow hold the protons together against their mutual repulsion, and that more and more neutrons are required to do this as the nucleus grows more massive? At first the number of neutrons required is roughly equal to the number of protons. The helium-4 nucleus contains 2 protons and 2 neutrons, the carbon-12 nucleus contains 6 protons and 6 neutrons, the oxygen-16 nucleus contains 8 protons and 8 neutrons, and so on. For more complicated nuclei, additional neutrons are needed. In vanadium-51, the nucleus contains 23 protons and 28 neutrons, five more than an equal amount. In bismuth-209, it is 83 protons and 126 neutrons, 43 more than an equal amount. For still more massive nuclei containing a larger number of protons, no amount of neutrons is sufficient to keep the assembly stable. The more massive nuclei are all radioactive. 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.