Since elementary school, we’ve learned that iron is the most abundant element on earth (32.1% by mass). The standard explanation was that iron (iron-56 specifically) was the “most stable nucleus.” However, I recently learned that Nickel-62 is actually the most tightly bound nucleus!
Although the nuclei of atoms consist only of neutrons and protons, simply summing the masses of these particles always yields a value less than the true mass. The missing mass is accounted for by the nuclear binding energy, a consequence of the strong nuclear force that attracts nucleons. Since energy is released as nucleons bind more tightly (by fusion for lighter elements and fission for heavier elements), one might expect Nickel-62 to be more common than iron-56. However, this is not the case, as evidenced by the data about the abundance of chemical elements in the Solar System below.
This conundrum can be partially resolved by noting that both iron-58 and nickel-62 do not have mass numbers divisible by 4. This means they cannot be produced by the alpha process. The alpha process is the process by which stars convert helium into heavier elements, essentially by adding alpha particles (He2+) to existing nuclides. Moreover, it seems that conditions in the stellar interior begin to favor photodisintegration over the alpha process around iron, ultimately leading to more iron-56 than nickel-62.
Tillaux's Thoughts 