High Atomic Number Isotopes and Radioactivity
Why are most isotopes of elements with a high
atomic number, radioactive?
The "real" answer to your question is complicated and not fully
understood. However, a reasonably good "explanation" is that as the
atomic number increases, one is trying to squeeze more positively
charged protons into the limited space of the atomic nucleus. A
consequence is that heavier mass isotopes, for example, U(238) is
more stable than U(235). The additional neutrons "dilute" the
coulombic repulsion. Consistent with this model is the observation
that high atomic number elements tend to decay by emitting an alpha
particle, thus reducing the nuclear charge by two protons. Of
course, in the "real world", things are more subtle, but this is
not a bad rule of thumb.
Thanks for your question.
As you know, the nucleus of an atom is composed of protons and neutrons.
The protons have a positive charge and because like charges repel, there is
a very strong electromagnetic force that would like for the protons to move
away from each other.
But another force, called the "strong force", works to keep the protons
together at very small distances in the nucleus. As the number of protons
increases, stable atomic nuclei also have an increasing number of neutrons
which bring "gluons" (the mediators of the strong force) to help keep the
nucleus together. But, as you can imagine, with more and more protons in
the nucleus, the chance that these high atomic number elements will release
energy (in the form of radiation) is greater.
Here are two websites that you may want to check out with additional
information. The first one is the "Chart of the Nuclides" which is a graph
of all isotopes with the atomic number (which identifies the element) on the
y-axis and the atomic mass number (which identifies the isotope) on the
There is also a website called the "Particle Adventure" which explains how
we know what we know about the particles that make up the "innards" of
Thanks for your question!
Todd Clark, Office of Science
US Department of Energy
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Update: June 2012