What about the nuclei of elements other than hydrogen?
All the other elements had nuclei more massive than that of hydrogen and
the natural first guess was that these were made up of some appropriate
number of protons closely packed together. The helium nucleus, which had
a mass four times as great as that of hydrogen, might be made up of 4
protons; the oxygen nucleus with a mass number of 16 might be made up of
16 protons and so on.
This guess, however, ran into immediate difficulties. A helium nucleus
might have a mass number of 4 but it had an electric charge of +2. If it
were made up of 4 protons, it ought to have an electric charge of +4. In
the same way, an oxygen nucleus made up of 16 protons ought to have a
charge of +16, but in actual fact it had one of +8.
Could it be that something was cancelling part of the positive electric
charge? The only thing that could do so would be a negative electric
charge[1] and these were to be found only on electrons as far as anyone
knew in 1914. It seemed reasonable, then, to suppose that a nucleus
would contain about half as many electrons in addition to the protons.
The electrons were so light, they wouldn’t affect the mass much, and
they would succeed in cancelling some of the positive charge.
Thus, according to this early theory, _now known to be incorrect_, the
helium nucleus contained not only 4 protons, but 2 electrons in
addition. The helium nucleus would then have a mass number of 4 and an
electric charge (atomic number) of 4 - 2, or 2. This was in accordance
with observation.
This “proton-electron theory” of nuclear structure accounted for
isotopes very nicely. While oxygen-16 had a nucleus made up of 16
protons and 8 electrons, oxygen-17 had one of 17 protons and 9
electrons, and oxygen-18 had one of 18 protons and 10 electrons. The
mass numbers were 16, 17, and 18, respectively, but the atomic number
was 16 - 8, 17 - 9, and 18 - 10, or 8 in each case.
Again, uranium-238 has a nucleus built up, according to this theory, of
238 protons and 146 electrons, while uranium-235 has one built up of 235
protons and 143 electrons. In these cases the atomic number is,
respectively, 238 - 146 and 235 - 143, or 92 in each case. The nucleus
of the 2 isotopes is, however, of different structure and it is not
surprising therefore that the radioactive properties of the
two—properties that involve the nucleus—should be different and that the
half-life of uranium-238 should be six times as long as that of
uranium-235.
The presence of electrons in the nucleus not only explained the
existence of isotopes, but seemed justified by two further
considerations.
First, it is well known that similar charges repel each other and that
the repulsion is stronger the closer together the similar charges are
forced. Dozens of positively charged particles squeezed into the tiny
volume of an atomic nucleus couldn’t possibly remain together for more
than a tiny fraction of a second. Electrical repulsion would send them
flying apart at once.
On the other hand, opposite charges attract, and a proton and an
electron would attract each other as strongly as 2 protons (or 2
electrons) would repel each other. It was thought possible that the
presence of electrons in a collection of protons might somehow limit the
repulsive force and stabilize the nucleus.
Second, there are radioactive decays in which beta particles are sent
flying out of the atom. From the energy involved they could come only
out of the nucleus. Since beta particles are electrons and since they
come from the nucleus, it seemed to follow that there must be electrons
within the nucleus to begin with.
The proton-electron theory of nuclear structure also seemed to account
neatly for many of the facts of radioactivity.
Why radioactivity at all, for instance? The more complex a nucleus is,
the more protons must be squeezed together and the harder, it would
seem, it must be to keep them together. _More and more electrons seemed
to be required._ Finally, when the total number of protons was 84 or
more, no amount of electrons seemed sufficient to stabilize the nucleus.
The manner of breakup fits the theory, too. Suppose a nucleus gives off
an alpha particle. The alpha particle is a helium nucleus made up, by
this theory, of 4 protons and 2 electrons. If a nucleus loses an alpha
particle, its mass number should decline by 4 and its atomic number by 4
- 2, or 2. And, indeed, when uranium-238 (atomic number 92) gives off an
alpha particle, it becomes thorium-234 (atomic number 90).
Suppose a beta particle is emitted. A beta particle is an electron and
if a nucleus loses an electron, its mass number is almost unchanged. (An
electron is so light that in comparison with the nucleus, we can ignore
its mass.) On the other hand, a unit negative charge is gone. One of the
protons in the nucleus, which had previously been masked by an electron,
is now unmasked. Its positive charge is added to the rest and the atomic
number goes up by one. Thus, thorium-234 (atomic number 90) gives up a
beta particle and becomes protactinium-234 (atomic number 91).
If a gamma ray is given off, that gamma ray has no charge and the
equivalent of very little mass. That means that neither the mass number
nor the atomic number of the nucleus is changed, although its energy
content is altered.
Even more elaborate changes can be taken into account. In the long run,
uranium-238, having gone through many changes, becomes lead-206. Those
changes include the emission of 8 alpha particles and 6 beta particles.
The 8 alpha particles involve a loss of 8 × 4, or 32 in mass number,
while the 6 beta particles contribute nothing in this respect. And,
indeed, the mass number of uranium-238 declines by 32 in reaching
lead-206. On the other hand the 8 alpha particles involve a decrease in
atomic number of 8 × 2, or 16, while the 6 beta particles involve an
increase in atomic number of 6 × 1, or 6. The total change is a decrease
of 16 - 6, or 10. And indeed, uranium (atomic number 92) changes to lead
(atomic number 82).
It is useful to go into such detail concerning the proton-electron
theory of nuclear structure and to describe how attractive it seemed.
The theory appeared solid and unshakable and, indeed, physicists used it
with considerable satisfaction for 15 years.
—And yet, as we shall see, it was wrong; and that should point a moral.
Even the best seeming of theories may be wrong in some details and
require an overhaul.