No simple rule. In general heavier ones are less stable, but a lot depends on the number of protons and neutrons present. Very heavy ones are all unstable.
How is one more massive than the other (Do heavy nuclei have more protons or nuetrons)?
They have more of both. Light stable nuclei have about the same number of protons and neutrons, the heavier the stable nucleus, the higher the neutron/proton ratio.