From: David Roberson
The above rule that I found makes it impossible to have two stable isotopes of elements with the same number of nucleons that are one level apart. An example of this rule would be that since He3 is stable, then H3 cannot be The possible exception is 40 nucleons. The “impossibility” depends on how precise you wording is. Is 10 billion years “stable”? If so, there is one exception. 40Ar is 99+ of all argon. Argon is element 18. Element 19 is potassium. 40K is radioactive, but with an extremely long half-life, over one billion years, so there is still primordial potassium on earth, in the natural mineral, and there will be a diminishing amount for billions of years in the future. In fact, there should be some primordial potassium here when out sun expires. That is relatively stable. So, to that extent 40K is both stable but radioactive. Of course, you can define “stable” to be “non-radioactive” but then you must take note that some grand unification theories (including the Sheldon Glashow original) predict that even the proton will decay eventually – making all mater radioactive in a long time enough time scale so that there are no stable and non-radioactive elements. Semantics is a bitch but 40 is magic. Which is to say that 40 is a magic number for nucleons – so much so that calcium, element 20 also has a stable 40 nucleon isotope. Bottom line there are three adjoining elements in the periodic table which all have isotopes of 40 a.m.u. and all retain at least an important percentage of that magic numbered isotope - from when our solar system formed 4-5 billion years ago.
