why is carbon-11 an unstable isotope?

Answer 1

C-11 has 6 protons and 5 neutrons in the nucleus. In general, there must be at least a number of neutrons equal to the protons (or even more neutrons) to give enough strong nuclear force to overcome the electric repulsion of the protons. Since there are fewer neutrons than proton there is not enough strong nuclear force to balance the repulsion of the protons.