The Most Tightly Bound Nuclei

The most tightly bound of the nuclei is ⁶²Ni, a case made convincingly by M. P. Fewell in an article in the American Journal of Physics. Though the championship of nuclear binding energy is often attributed to ⁵⁶Fe, it actually comes in a close third. The four most tightly bound nuclides are listed in the table below with a tabulation of the binding energy B divided by the mass number A. The curve adapted from Fewell shows those nuclides that are close to the peak.

Nuclide B/A (keV/A) 62Ni 8794.60 +/- 0.03 58Fe 8792.23 +/- 0.03 56Fe 8790.36 +/- 0.03 60Ni 8780.79 +/- 0.03 Data from Wapstra and Bos.

The most tightly bound nuclides are all even-even nuclei. The curve drawn through the cluster of nuclei above is just to show the nature of the trend with mass number. A similar kind of trend is observed with even-odd nuclides at a lower range of binding energy, and then by odd-odd nuclei at the least-bound extreme. The peaks of all three groups occur around A = 60.

The high binding energy of this group of elements around A=60, typically called "the iron group" by astrophysicists, is significant in the understanding of the synthesis of heavy elements in the stars. It is curious that the abundance of ⁵⁶Fe is an order of magnitude higher than that of ⁶²Ni. Fewell discusses this point, and indicates that the reason lies with the greater photodisintegration rate for ⁶²Ni in stellar interiors.