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.
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.)
[Illustration: _Werner Heisenberg_]
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 first 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 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.