Nickel-62

Nickel-62 is an isotope of nickel having 28 protons and 34 neutrons.

It is a stable isotope, with the highest binding energy per nucleon of any known nuclide (8.7945 MeV). It is often stated that 56Fe is the "most stable nucleus", but only because 56Fe has the lowest mass per nucleon (not binding energy per nucleon) of all nuclides. The lower mass per nucleon of 56Fe is possible because 56Fe has 26/56 ≈ 46.43% protons, while 62Ni has only 28/62 ≈ 45.16% protons. Protons are less massive than neutrons, meaning that the larger fraction of protons in 56Fe lowers its mean mass-per-nucleon ratio in a way that has no effect on its binding energy.

Properties
The high binding energy of nickel isotopes in general makes nickel an "end product" of many nuclear reactions (including neutron capture reactions) throughout the universe and accounts for the high relative abundance of nickel—although most nickel in space (and thus produced by supernova explosions) is nickel-58 (the most common isotope) and nickel-60 (the second-most), with the other stable isotopes (nickel-61, nickel-62, and nickel-64) being quite rare. This suggests that most nickel is produced in supernovas in the r-process of neutron capture from nickel-56 immediately after the core-collapse, with any nickel-56 that escapes the supernova explosion rapidly decaying to cobalt-56 and then stable iron-56.

Relationship to iron-56
The second and third most tightly bound nuclei are those of $8,794.553$Fe and $58$Fe, with binding energies per nucleon of 8.7922 MeV and 8.7903 MeV, respectively.

As noted above, the isotope $56$Fe has the lowest mass per nucleon of any nuclide, 930.412 MeV/c$56$, followed by $2$Ni with 930.417 MeV/c$62$ and $2$Ni with 930.420 MeV/c$60$. As noted, this does not contradict binding numbers because $2$Ni has a greater proportion of neutrons which are more massive than protons.

If one looks only at the nuclei, without including the electrons, $62$Fe again shows the lowest mass per nucleon (930.175 MeV/c$56$), followed by $2$Ni (930.181 MeV/c$60$), and $2$Ni (930.187 MeV/c$62$).

The misconception of $2$Fe's higher nuclear binding energy probably originated from astrophysics. During nucleosynthesis in stars the competition between photodisintegration and alpha capturing causes more $56$Ni to be produced than $56$Ni ($62$Fe is produced later in the star's ejection shell as $56$Ni decays). The $56$Ni is the natural end product of silicon-burning at the end of a supernova's life and is the product of 14 alpha captures in the alpha process which builds more massive elements in steps of 4 nucleons, from carbon. This alpha process in supernovas burning ends here because of the higher energy of zinc-60, which would be produced in the next step, after addition of another "alpha" (or more properly termed, helium nucleus).

Nonetheless, 28 atoms of nickel-62 fusing into 31 atoms of iron-56 releases $56$ of energy; hence the future of an expanding universe without proton decay includes iron stars rather than "nickel stars".