The Story of Nuclear Energy, Volume 2 (of 3): The Proton-Neutron Theory

As soon as the neutron was discovered, the German physicist Werner Karl Heisenberg (1901- ) revived the notion that the nucleus must be made up of protons and neutrons, rather than protons and electrons. It was very easy to switch from the latter theory to the former, if one simply remembered to pair the electrons thought to be in the nucleus with protons and give the name neutrons to these combinations.

Thus, the helium-4 nucleus, rather than being made up of 4 protons and 2 electrons, was made up of 2 protons and 2 proton-electron combinations; or 2 protons and 2 neutrons. In the same way the oxygen-16 nucleus instead of being made up of 16 protons and 8 electrons, would be made up of 8 protons and 8 neutrons.

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The proton-neutron theory would account for mass numbers and atomic numbers perfectly well. If a nucleus was made up of x protons and y neutrons, then the atomic number was equal to x and the mass number to x + y. (It is now possible to define the mass number of a nucleus in modern terms. It is the number of protons plus neutrons in the nucleus.)

The proton-neutron theory of nuclear structure could account for isotopes perfectly well, too. Consider the 3 oxygen isotopes, oxygen-16, oxygen-17, and oxygen-18. The 100first would have a nucleus made up of 8 protons and 8 neutrons; the second, one of 8 protons and 9 neutrons; and the third, one of 8 protons and 10 neutrons. In each case the atomic number is 8. The mass numbers however would be 16, 17, and 18, respectively.

In the same way uranium-238 would have a nucleus built of 92 protons and 146 neutrons, while uranium-235 would have one of 92 protons and 143 neutrons.

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 101as 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.