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Re: radioactivity



Hi all-

On Wed, 25 Oct 2000, cliff parker wrote:

I will expose my ignorance with this question but here goes. I
have been thinking about radioactivity. Some of my high school
chemistry students asked me why elements with nuclei larger than
uranium were radioactive.
If the question assumes that radioactivity is a property only of
the very heavy elements, the assumption is incorrect.
Elements fly apart - emitting "stuff" - whenever energy
conservation permits them to do so.
A nucleus in an excited state will decay to a lower state by
gamma emmission.
A nucleus will decay to another more stable nucleus of the
same A and charge different by one unit by beta emission.
A nucleus will split into nuclei of two or more different elements
if the split is energetically possible (think of the neutron and proton
as "elements" in this context).

I said that the electromagnetic force
causes positive protons to repel each other while the strong
nuclear force holds protons and neutrons together. The strong
force is stronger than electromagnetic force and wins the battle
when nuclei are formed.

You are really answering a different question - why are some
nuclei more stable than others. You should clarify the difference to
the students.

The strong force however has limited
range and when nuclei reach about the size of U the strong force
can no longer attract the particles with enough force to overcome
electromagnetic repulsion therefor such large nuclei do not last
long.

This explanation is incorrect, or at least, incomplete. Think of
a chain of students holding hands. The force is short range - two arm
lengths - but there is no limit to the length of a possible chain.
You are correct in pointing to competition between the coulomb
force (proportional to Z^2) and the nuclear force (proportional to A)
as limiting the number of protons. This leads to the existence of a
proton "drip line". There is also a neutron "drip line" - possibly
because of the Pauli principle limiting the number of neutrons that
can be near the center. So having the right mix of n and p is crucial.

I believe in as far as my explanation goes it is correct.
However I am sure there is more because this explanation says
nothing about why lighter nuclei are radioactive. It also says
nothing about why heavy nuclei have varying half-lifes. My guess
is that most of the rest of the story lies with the weak force
which I know little about. Will some of my cyber mentors shed
some light and or point me in the right direction for I fear that
in the process of explaining this to my students, I may have
"expressed myself more clearly than I can think" and I hate it
when that happens!

Note that among the light elements those with equal n and p
numbers tend to be the most stable. Also, there is a shell structure
so that neutron or proton numbers 2, 6, 8, ... are the most stable.

There are some good texts, I don't have any of them handy as I
write.
Regards,
Jack