http://www.ck12.org Chapter 25. Nuclear Physics
25.2. The maximum binding energy per nucleon occurs for iron atoms. Generally, the atoms in the periodic table
(Figure25.7) before iron are formed by the process of fusion within stellar interiors. Beyond iron, the greater
energies needed to fuse nuclei result from stellar events called supernovas. Those atoms beyond iron are generally
less stable, and decay through the process of fission. Fission occurs naturally through the process of radioactive
decay, which we will shortly discuss, and can be induced artificially in nuclear reactors and nuclear bombs.
FIGURE 25.2
Average binding energy per nucleon
(MeV) vs. atomic mass number, A.Notice that the binding energy of the nucleus of an atom is much greater than the binding energy of the electrons held
by the nucleus. Earlier, we saw that the energy needed to ionize a hydrogen atom–that is, to separate the ground-state
electron– was only 13.6eV.
The strong nuclear force and radioactivity
Physicists realized that another force must exist if protons could be in such close proximity within a nucleus and
not fly apart due to electrostatic repulsion. Indeed, this force must be considerably stronger than the electrostatic
force, or stable nuclei could not exist. We recognize this force today as one of the four fundamental forces in nature:
the strong nuclear force. Many details about the nuclear strong force are not known yet, but we do know that it
is a very short-range attractive force. It does not exist outside the nucleus. Within a nucleus, protons attract other
protons, neutrons attract other neutrons, and they attract one another. As we say, the strong nuclear force is charge-
independent. The strong nuclear force is some hundred times greater than the electrostatic force, but only if the
nucleons are no farther apart than about 10−^15 m.
For the nuclei of the elements that follow iron in the periodic table, neutrons outnumber protons. Generally–the
heavier the nucleus, the greater the proportion of neutrons. Uranium, the largest naturally occurring atom, due to its
many isotopes has about 50—55 more neutrons than protons.
Remember that the repulsive electrostatic force between the protons is countered by the attractive strong nuclear
force between nucleons. But the strong force is very short-ranged, so its effect hardly extends beyond two adja-
cent nucleons. The electrostatic force, however, extends throughout the entire atom (in principle, out to infinity).
Therefore, there is a limit to the number of protons that can exist within a nucleus before the stability of the nucleus
is compromised. The more nucleons there are, the larger the nucleus becomes. As this happens, the short-range
strong nuclear force weakens (very abruptly) and the repulsive electrostatic force dominates. Neutrons, however,
are not subject to electrostatic repulsion butaresubject to the strong force attraction. Thus, they help to keep the
nucleus together without adding to the electrostatic repulsion. Still, almost all nuclei with more than 82 protons are
unstable (exhibit radioactivity). We will soon see that unstable nuclei can be transformed into entirely different ones