Binding energy per nucleon decreases with $A$ for nuclei with atomic mass number $A > 100$ due to weak nuclear forces. It can be explain as : At short distances, the nuclear force is stronger than the Coulomb force; it can overcome the Coulomb repulsion of protons inside the nucleus. At typical nucleon separation $(1.3\, fm)$ it is a very strong attractive force ($104$ $newtons$ ). 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 $(2.5\, fm, A > 100)$.