Re: radioactivity

["John M. Clement" <clement@HAL-PC.ORG>]

Ludwig gave a good answer, but I think that your answer would be good for students in HS.  You can point out that your answer also is a good model for why there are "excess" neutrons in heavier nucleii.  Protons repel, but Neutrons do not.

Unfortunately a better explanation could be very complex.  If you wish to make a stab at it you could say that nucleons behave in a similar fashion to the electrons in an atom.  Because of quantization, they occupy definite orbits or shells in pairs.  This means that certain numbers of neutrons and or protons will be in closed shells and will have lower energy.  Adding an extra neutron increases the energy, as it must go into a higher energy shell.

This appeal to Chemistry ideas may help to satisfy some students.  If they hated Chemistry, and found it difficult, this will not be helpful.  If they are very interested you might send them to the chapter on Nuclear Structure in Asimov's "Understanding Physics: The Electron, Proton, and Neutron".

John M. Clement


> 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.  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.  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.  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!
>
> --
> Cliff Parker
>
> Never express yourself more clearly than you can think.  --
> Niels Bohr