Alpha nuclides have equal, even numbers of protons and neutrons; they are important in stellar nucleosynthesis since the energetic environment within stars is amenable to fusion of alpha particles into heavier nuclei.
[1][2] Stable alpha nuclides, and stable decay products of radioactive alpha nuclides, are some of the most common metals in the universe.
[3] The entries for 36Ar and 40Ca are theoretical: they would release energy on decay, but the process has never been observed, and the half-lives are probably extremely long.
Likewise, the chains for masses 64, 84, 92, and 96 theoretically can continue one more step by double electron capture (to 64Ni, 84Kr, 92Zr, and 96Mo respectively), but this has never been observed.
As of 2024[update], the heaviest known alpha nuclide is xenon-108.