[jcjlf]

Who can give an explanation at secondary level of the fact that (except for some lower atomic numbers like H-2, Li-6, B-10 and N-14) isotopes with an uneven number of protons combined with an uneven number of neutrons don't occur in nature above Z = 7? Has this anything to do by building up the elements by alpha particles?

[lancenti]

I've never actually come across such a problem, but I would suspect it is a stability issue.

However, naturally-occurring may need to be defined more precisely. Manganese-54 and Manganese-46 do exist albeit for a very short time.

[Borek]

Has this anything to do by building up the elements by alpha particles?

I hoped Mitch will answer, but seems he is busy finishing his PhD.

No, idea of alpha particle as a building block is wrong. However, there are models of nucleus, a little bit similar to the electron configuration of an atom. And just as in the atom some configurations (those with paired spins) have usually lower energy, nuclei having even numbers of protons/neutrons are more stable.

Could be I remember it completely wrong.