What Rutherford did not (and could not) take into account were the
consequences of a completely new type of nuclear bombardment involving a
type of particle unknown in the 1920s (though Rutherford speculated
about the possibility of its existence).
The beginnings of the new path came about through the reluctant
realization that there was a flaw in the apparently firmly grounded
proton-electron picture of nuclear structure.
The flaw involved the “nuclear spin”. In 1924 the Austrian physicist
Wolfgang Pauli (1900-1958) worked out a theory that treated protons and
electrons as though they were spinning on their axes. This spin could be
in either direction (or, as we would say in earthly terms, from
west-to-east, or from east-to-west). Quantum theory has shown that a
natural unit exists for what is called the angular momentum of this
spin. Measured in terms of this natural unit of spin, the proton and the
electron have spin ½. If the particle spun in one direction it was +½,
if in the other it was -½.
When subatomic particles came together to form an atomic nucleus, each
kept its original spin, and the nuclear spin was then equal to the total
angular momentum of the individual particles that made it up.
For instance, suppose the helium nucleus is made up of 4 protons and 2
electrons, as was thought in the 1920s. Of the 4 protons, suppose that
two had a spin of +½ and two of -½. Suppose also that of the 2
electrons, one had a spin of +½ and one of -½. All the spins would
cancel each other. The total angular momentum would be zero.
Of course, it is also possible that all 6 particles were spinning in the
same direction; all +½ or all -½. In that case the nuclear spin would be
3, either in one direction or the other. If 5 particles were spinning in
one direction and 1 in the other, then the total spin would be 2, in one
direction or the other.
[Illustration: _Wolfgang Pauli lecturing in Copenhagen in April 1929._]
In short if you have an even number of particles in a nucleus, each with
a spin of +½ or -½, then the total spin is either zero or a whole
number, no matter what combination of positive and negative spins you
choose. (The total spin is always written as a positive number.)
On the other hand, suppose you have lithium-7, which was thought to be
made up of 7 protons and 4 electrons. If the 7 protons were all +½ and
the 4 electrons were all -½ in their spins, the nuclear spin would be
⁷/₂ - ⁴/₂ = ³/₂.
If you have an odd number of particles in the nucleus, you will find
that any combination of positive and negative spins will _never_ give
you either zero or a whole number as a sum. The sum will always include
a fraction.
Consequently, if one measures the spin of a particular atomic nucleus
one can tell at once whether that nucleus contains an even number of
particles or an odd number.
This quickly raised a problem. The nuclear spin of the common isotope,
nitrogen-14, was measured accurately over and over again and turned out
to be 1. There seemed no doubt about that and it could therefore be
concluded that there were an even number of particles in the nitrogen-14
nucleus.
And yet, by the proton-electron theory of nuclear structure, the
nitrogen-14 nucleus, with a mass number of 14 and an atomic number of 7,
had to be made up of 14 protons and 7 electrons for a total of 21
particles altogether—an odd number.
The nuclear spin of nitrogen-14 indicated “even number” and the
proton-electron theory indicated “odd number”. One or the other had to
be wrong, but which? The nuclear spin was a matter of actual
measurement, which could be repeated over and over and on which all
agreed. The proton-electron theory was only a theory. It was therefore
the latter that was questioned.
What was to be done?
Suppose it is wrong to count protons and electrons inside the nucleus as
separate particles. Was it possible that an electron and a proton,
forced into the close confinement of the atomic nucleus might, by the
force of mutual attraction, become so intimately connected as to count
as a single particle. One of the first to suggest this, as far back as
1920, was Rutherford.
Such a proton-electron combination would be electrically neutral and in
1921 the American chemist William Draper Harkins (1873-1951) used the
term “neutron” as a name for it.
If we look at the nitrogen-14 nucleus in this way then it is made up,
not of 14 protons and 7 electrons, but of 7 protons and 7
proton-electron combinations. Instead of a total of 21 particles, there
would be a total of 14; instead of an odd number, there would be an even
number. The structure would now account for the nuclear spin.
But could such a revised theory of nuclear structure be made to seem
plausible? The proton-electron theory seemed to make sense because both
protons and electrons were known to exist separately and could be
detected. If an intimate proton-electron combination could also exist,
ought it not exist (or be made to exist) outside the nucleus and ought
it not be detected as an isolated particle?