For Heavy Elements What Is The Ratio Of Neutrons To Protons That Predicts A Stable Nucleus

for a heavy nuclei with atomic number above 20, the ration for neutron to proton is 1.5: 1. in heavy nucleus the number of neutron is greater than the number of protons,  this is necessary  in order to overcome the repulsive coulomb force between the protons, the more the number of protons increases the greater the coulomb force, hence excess neutron are needed to forfeit this effect.

 this why isotopes  with neutron to proton number less than unity are naturally unstable and must subsequently decay to attend stability. this isotopes have  more protons than neutrons , this excess protons must be shade off for them to attend stability. therefore unstable isotopes will either decay through alpha, beta or positron emission to attend stability.