My understanding of what makes an atom unstable is this: Large nuclei will have a high number of protons, and as a result there will be large amount of electrostatic repulsion between the protons in the nucleus. So when the nucleus is too large (too many protons) The electrostatic repulsive forces between the protons is greater than the strong nuclear forces that are acting between quarks in neighboring nucleons. (The strong force is obviously strong in each individual nucleon, but is less strong between the quarks which are not in the same nucleon) So when an atom needs to become more stable, it needs to reduce the size of its nucleus, and it can do this via alpha decay. Or it can reduce the repulsive forces present in the nucleus by undergoing beta plus decay, which changes an up quark to a down quark, and as a result turns a proton into a neutron. (But Beta minus decay turns a neutron to proton, so isnt that particular decay a step towards instability?) Would be grateful if someone could make this clear for me. Thanks in advance.