The chemical elements beyond lead are radioactive, and they do not have stable isotopes. This means that they will decay into other elements. Except for Plutonium, their half life is in the order of several minutes, to seconds. There is a theory in physics that says that after a number of elements with short half lives, there will be others with longer half lives. These are generally known as Islands of stability. These elements are expected to have isotopes with half lives in the range of several minutes. The hypothesis is that the atomic nucleus is built up in "shells" in a manner similar to the structure of the much larger electron shells in atoms. In both cases, shells are just groups of quantum energy levels that are relatively close to each other. Energy levels from quantum states in two different shells will be separated by a relatively large energy gap. So when the number of neutrons and protons completely fills the energy levels of a given shell in the nucleus, the per nucleon will reach a local maximum and thus that particular configuration will have a longer lifetime than nearby isotopes that do not possess filled shells.