Why is the Earth made of iron-56 and not nickel-62?

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.

Binding energy per nucleon. Iron-56 is actually in 3rd place! Figure from http://hyperphysics.phy-astr.gsu.edu/ which was adapted from a paper by M.P. Fewell
Abundance of chemical elements in the Solar system. Note the logarithmic scale and the peak at iron. Figure from https://en.wikipedia.org/wiki/Iron_peak

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.


  1. http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/nucbin.html#c1
  2. https://en.wikipedia.org/wiki/Iron_peak
  3. http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/nucbin2.html#c1
  4. https://en.wikipedia.org/wiki/Iron-56
  5. https://en.wikipedia.org/wiki/Nickel-62

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: