How do you know what will undergo alpha, beta minus, or beta plus decay?
Beta decay occurs in nuclei with an excess of neutrons. These can be very light: Tritium which is hydrogen with 2 neutrons beta decays with a rather short half life (12 years I think) but of course it also occurs in heavy nuclei. Excess neutrons are weakly bound, making their effective mass not very different from that of free neutrons and making them prone to beta decay. Beta+ decay occurs in neutron poor nuclei. The stronger electrostatic repulsion between protons in these nuclei (stronger since their average separation is lowered by the lack of neutrons) increases the proton effective mass and if this surpasses the neutron effective mass, then beta+ decay can occur. Alpha decay only occurs in heavy nuclei, where the binding energy per nucleon is lowered with respect its Fe-Co-Ni maximum It occurs by tunnelling through the potential barrier which is the result of the strong nuclear force and the electrostatic force combined. At short distances the strong force always prevail-it is at
