If the neutron/proton ratio of an atom is too high then the isotope is radioactive?

likewise if the ratio is too low then the isotope is considered as radioactive as well. So my question is, how do you know if the ratio is too high or too low. For example, Chlorine has 17 protons and it has 18 neutrons. All in all my question is what are the numbers for the ratio of neutron to proton that is considered low/good/high. Thanks for all your help