Island of stability
The chemical elements beyond lead are radioactive, and they do not have stable isotopes. This means that they will decay into other elements. As their atomic number (the number of protons in their nucleus) increases, the time it takes for them to decay tends to get shorter, going from billions of years to less than one millisecond. 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 that decay in several minutes.The theory is that, similarly to electron shells, the nucleus of an atom is made of "shells" (which can be thought of as layers of protons and neutrons), and that when a nucleus has entirely filled shells, it is significantly more stable.