226 The Poetry of Physics and The Physics of Poetry
hydrogen nucleus but a charge of only +2e. Before the discovery of the
neutron it was believed that the helium nucleus consisted of 4 protons
and 2 electrons. It was only after the discovery of the neutron in 1930
that it was realized that the helium nucleus is composed of two protons
and two neutrons. The proton and neutron are referred to generically as
nucleons. This term is used to describe the two particles because their
interactions within the nucleus are more or less the same with only minor
corrections because of the difference of their charges. The mass number
of a nucleus, A, is equal to the total number of nucleons. The atomic
number, Z, is equal to the total number of protons and hence the total
number of electrons. The total number of neutrons in the nucleus, N,
equals A-Z. The mass of the nucleus is approximately equal to MA
where M is the mass of a nucleon (the mass of the proton and a neutron
are approximately equal). The charge of the nucleus is exactly equal to
+Ze.
Not all nuclei are stable. Some are radioactive which means that they
spontaneously change into another nucleus through the emission of an α
particle, an electron, a positron or a neutron. It was through the study of
the chemistry of radioactive nuclei that it was discovered that there are a
number of nuclei with the same value of Z but a different value of A.
Two nuclei with the same number of protons but a different number of
neutrons are called isotopes. Most elements consist of a number of
isotopes as any inspection of the periodic table reveals. The existence of
isotopes explains why the mass of all elements is not an even integer
times the mass of hydrogen. The element chlorine has the atomic weight
of 35.5 because 80% of the element is Cl 35 and 20% Cl 37. Both Cl 35 and
Cl 37 consist of 17 protons but Cl 37 has 20 neutrons while Cl 35 has only
- An atom of Cl 35 and an atom of Cl 37 behave identically from a
chemical point of view despite the fact that one is heavier than the other.
The same is true of uranium-235 and uranium-238; they are identical
chemically. Their nuclei behave quite differently, however. The lighter
isotope is much more radioactive than the heavier one. U 235 participates
much more readily in the process of nuclear fission than does U 238.
The Nuclear Force
The very first question that the existence of nuclei raises is the nature of
their stability. The protons of the nucleus all have the same charge and
hence electrically repel each other. The strength of this repulsion is quite