Some nuclides are unstable, meaning they have a limited lifespan. An unstable nucleus
is called a radionuclide, and it will spontaneously and rapidly decay or split up into more
stable pieces. Such materials are radioactive, and the details of the decay process are
the subject of another section.
In this section, we discuss what makes for a stable nucleus. The question of stability is
of fundamental importance. If all elements were unstable, then life would as we know it
would not exist í the carbon, oxygen and other elements that make up your body would
be constantly changing into other elements. Life is complicated enough!
On the other hand, if all elements were equally stable, then the nuclear fusion process
that powers stars (including the Sun) would never happen, and the universe would be
almost all hydrogen, with none of the heavier elements that make life possible.
To explain why some atoms are stable and others are not, it helps to consider a
diagram of stable and unstable nuclides where Z is plotted against N. For example,
even though isotopes of silver with its 47 protons (Z = 47) have been created with
mass numbers as low as 96 and as high as 124, just two of these nuclides are stable (
A = 107 and 109). In other words, there must be 60 or 62 neutrons along with the 47
protons.
We would like you to observe three important features of this diagram. First, note that the stable nuclides are clustered around a band running
through the diagram. The unstable nuclides exist on either side of this band. Second, you can see how the stable nuclei are distributed.
Roughly speaking, for less massive nuclei, the number of protons and neutrons is approximately equal: They cluster around the line N = Z. In
contrast, for more massive nuclei, the number of neutrons exceeds the number of protons, N > Z. Third, observe that there are no stable
nuclei beyond bismuth (Z = 83).
Since neutrons are effective at diluting the repulsive electric force between protons (by spacing them out more), and the strong force binds
neutrons effectively to protons and other neutrons, it seems like having more neutrons can only bind the nucleus more tightly. You may be
wondering why an atom cannot have an extremely high ratio of neutrons to protons. For instance, why are there no hydrogen isotopes with
eight neutrons, or even just seven neutrons? Quantum mechanical principles dictate why.
In contrast, explaining why very large nuclei are unstable, even if N is closer to Z, only requires considering the nature of the strong and
electrostatic forces, not quantum mechanics.
Stable nuclei
Stable only for certain combinations of
neutrons and protonsUnstable nuclei
Other combinations do not exist or are
unstableGraph of stable and unstable
nuclei
Values clustered near “band of stability”
Ratio of neutrons to protons increases
with nuclear size
Larger nuclei: more neutrons required to
dilute protons