more stable the nucleus is. The graph has its maximum of 8.8 MeV/nucleon when the
total number of nucleons is 56. The nucleus that has 56 protons and neutrons is^5626 Fe,
an iron isotope. This is the most stable nucleus of them all, since the most energy is
needed to pull a nucleon away from it.
Two remarkable conclusions can be drawn from the curve of Fig. 11.12. The first
is that if we can somehow split a heavy nucleus into two medium-sized ones, each of
the new nuclei will have morebinding energy per nucleon than the original nucleus
did. The extra energy will be given off, and it can be a lot. For instance, if the uranium
nucleus^23592 U is broken into two smaller nuclei, the binding energy difference per
nucleon is about 0.8 MeV. The total energy given off is therefore0.8 (235 nucleons)188 MeV
This is a truly enormous amount of energy to be produced in a single atomic event.
As we know, ordinary chemical reactions involve rearrangements of the electrons in
atoms and liberate only a few electronvolts per reacting atom. Splitting a heavy nucleus,
which is called nuclear fission,thus involves 100 million times more energy per atom
than, say, the burning of coal or oil.
The other notable conclusion is that joining two light nuclei together to give a single
nucleus of medium size also means more binding energy per nucleon in the new nucleus.
For instance, if two^21 H deuterium nuclei combine to form a^42 He helium nucleus, over 23
MeV is released. Such a process, called nuclear fusion,is also a very effective way to ob-
tain energy. In fact, nuclear fusion is the main energy source of the sun and other stars.
The graph of Fig. 11.12 has a good claim to being the most significant in all of sci-
ence. The fact that binding energy exists at all means that nuclei more complex than
the single proton of hydrogen can be stable. Such stability in turn accounts for the
existence of the elements and so for the existence of the many and diverse forms of
matter we see around us (and for us, too). Because the curve peaks in the middle, we
have the explanation for the energy that powers, directly or indirectly, the evolution of
the universe: it comes from the fusion of light nuclei to form heavier ones.Example 11.5
(a) Find the energy needed to remove a neutron from the nucleus of the calcium isotope^4220 Ca.
(b) Find the energy needed to remove a proton from this nucleus. (c) Why are these energies
different?MeV
nucleon402 Chapter Eleven
T
he short-range attractive forces between nucleons arise from the strong interaction.(There
is another fundamental interaction affecting nucleons called the weak interactionthat will
be discussed in Chaps. 12 and 13.) The strong interaction is what holds nucleons together to
form nuclei, and it is powerful enough to overcome the electric repulsion of the positively charged
protons in nuclei provided neutrons are also present to help. If the strong interaction were a
little stronger—perhaps only 1 percent would be enough—two protons could stick together
without any neutrons needed. In this case, when the universe came into being in the Big Bang
(Sec. 13.8), all its protons would have joined into diprotons almost as soon as they appeared.
Then there would be no individual protons to undergo the fusion reactions that power the stars
and have created the chemical elements. The universe would be a very different place from what
it is today, and we would not exist.The Strong Interaction
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