Re: radioactivity

[John Denker <jsd@MONMOUTH.COM>]

At 08:41 PM 10/25/00 -0500, cliff parker wrote:
> Some of my high school chemistry students asked me why elements with
> nuclei larger than uranium were radioactive.

Cool.  Interesting question.

> I said that the electromagnetic force causes positive protons to repel
> each other while the strong nuclear force holds protons and neutrons
> together.

Yes.

> The strong force is stronger than electromagnetic force and wins the battle
> when nuclei are formed.  The strong force however has limited range

Yes.

> 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.

Also don't forget that the electrostatic piece grows like the _square_ of
the atomic number (divided by nuclear radius).  That means high-Z nuclides
are heavily penalized.

> I believe in as far as my explanation goes it is correct.

Right.  There are of course additional details.

The Encyclopedia Britannica has a nice writeup on "nuclear models" in the
ATOMS article.

Also a refinement:  It would have been cleverer for the student to ask why
all nuclides heavier than bismuth (Z=83 A=209) are radioactive.
  http://klbproductions.com/yogi/periodic/Bi-pg2.html
Uranium(92, 238) is long-lived enough to have a nonzero natural abundance,
but it is still measurably radioactive
  http://klbproductions.com/yogi/periodic/U-pg2.html



> 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.

Yes, weak interactions are part of the story.  Weak decays are common among
light and heavy nuclides both.

By the same token, nonweak decays are relevant for light and heavy nuclides
both.  For instance, the shell model predicts there will never be a stable
helium-5 or lithium-5:  ones are stable and fours are stable... fives don't
have a chance.