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Name: Michael
Status: student
Grade: 9-12
Location: NJ
Country: N/A
Date: 1/20/2006

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.

Vince Calder


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 x-axis:

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 subatomic particles:

Thanks for your question!

Todd Clark, Office of Science
US Department of Energy

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