Binding energy per nucleon decreases with for nuclie with the atomic mass number due to weak nuclear forces. At short distances, the nuclear force is stronger than Coulomb force; it can overcome the coulomb repulsion of protons inside nucleus. At typical nucleon separation (1.3 fm) it is very strong attractive force . Beyond about 1.3 fm separation, the force exponentially dies off to zero. However, the coulomb force between protons has a much larger range and becomes the only significant force between protons when their separation exceeds about ( )

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