CBSE Class 10 Answered
Nuclei are bound together by the residual strong force. The residual strong force is minor residuum of the strong force which binds quarks together to form protons and neutrons. This force is much weaker between neutrons and protons because it is mostly neutalized within them, in the same way that electromagnetic forces between neutral atoms (van der Waals forces) are much weaker than the electromagnetic forces that hold the atoms internally together.
THowever, because the residual strong force has a limited range because it decays quickly with distance , only nuclei smaller than a certain size can be completely stable. The largest known complete stable nucleus is lead-208 which contains 208 neutrons and protons. Nuclei larger than this maximal size of 208 particles generally become increasingly short-lived, as the number of neutrons and protons which compose them increases beyond this number. The "ratio" of protons to neutrons in a nucleus is intrinsically unstable.