What makes an isotope stable? I have heard many times that when there is the “right balance” of protons and neutrons the atom will be stable, and therefore it will decay to achieve this stability. But what about this supposed balance of protons and neutrons makes it stable? Does it have to do with quarks? Density? I am quite curious as to what causes this.
There is a shell structure within the nucleus also. So the neutron/proton ratio varies from 1:1 for the light elements to 1.5 for the heavy elements. 




  Note: I ripped off the image from another website. Not sure where they got it from...