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As the strong nuclear force is a short range force, and as it is way stronger than Coulomb force, Lumen Learning answered my question as follows: “for low-mass nuclei, the nuclear attraction dominates and each added nucleon forms bonds with all others, causing progressively heavier nuclei to have progressively greater values of BE/A.” But according to wikipedia:

  1. “a neutron has a root mean square radius of about 0.8 fm,” and “the root mean square radius of a proton is about 0.84–0.87 fm.” which means that the diameter of a nucleon is definitely less than 2 fm.

  2. “the nuclear force is powerfully attractive between nucleons at distances of about 1 fm but it rapidly decreases to insignificance at distances beyond about 2.5 fm.” which means that the strong nuclear force of a nucleon -surrounded by nucleons- has an effect only on nucleons next to it, and it doesn’t really have any significant effect on nucleons beyond those, i.e. a nucleon doesn’t really form bonds with all other nucleons in medium-mass nuclei. So, why does the BE/A rise with the rise of A, then start dropping away.

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The reason the trend reverses after iron is the growing positive charge of the nuclei, which tends to force nuclei to break up. It is resisted by the strong nuclear interaction, which holds nucleons together. The electric force may be weaker than the strong nuclear force, but the strong force has a much more limited range: in an iron nucleus, each proton repels the other 25 protons, while the nuclear force only binds close neighbors. So for larger nuclei, the electrostatic forces tend to dominate and the nucleus will tend over time to break up.

italics mine

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