To begin the qualitative explanation you must understand what types of
nuclear decay occur, and study the stability factors for each type of decay.
Eric keeps asking why Tc (Z = 43) has no stable isotopes (and also Pm with Z
of 61) when all the other elements with no stable isotopes are Z >= 84.
On the surface this is a valid and interesting question. However, once you
realize that the Z >= 84 elements (to which Eric wants to compare
technetium) are alpha unstable, but the technetium isotopes are beta
unstable, then continued comparison becomes pointless. (Sorry Eric.) It's
pointless because the things that determine beta instability and the things
that determine alpha instability are different. Beta decay and alpha decay
are very, very different processes. Beta decay is a weak-nuclear-force
thing and alpha decay is a strong-nuclear-force thing. For nuclides near
beta stability, alpha decay begins to be important and eventually
predominates around Z of 84. Beta-decay predominates at the lower Z
values.
There is absolutely nothing remarkable about Tc having no stable isotopes on
the basis of it having low Z. Beta decay doesn't have anything to do with
how large Z is. It does have something to do with the Z:N ration, and
whether Z is even or odd... but not whether Z is large or small.
There are over 7000 cultivars of apples. A quick check seems to show less
than 100 cultivars of oranges. Why are there more apples than oranges? I
don't know, but I don't have any particular reason to think that just
because these are both cultivated fruits that there should be relatively
equal numbers of apple and orange varieties. Thinking this is unusual and
trying to explain it might be a case of comparing apples to oranges.
Likewise using an alpha-decay criterion (big Z) for stability questions in
the beta decay region (lower Z) is like comparing apples to oranges. If Tc
had no stable isotopes because all Tc isotopes decay by alpha decay, *that*
would indeed stick out like a sore thumb. But since all Tc isotopes decay
by beta decay, they are just part of the bountiful sea of nuclides in that
region that also decay by beta decay. There is nothing unusual about them.
Is Eric a deluded chemist? Only in the sense that he is thinking too much
in terms of Z (which is appropriate for a typical chemist) and not thinking
enough in terms of N and A, which would be delusional behavior for a nuclear
chemist/physicist. For the bulk of nuclides (which are beta unstable) we
need to look at both Z and N (from which A follows), and we should group
things by (Z,N) or (Z,A) or (N,A) or just A. Grouping by Z is rightly a
chemistry thing. It is not a nuclear thing.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu