Island of stability
A filled shell would have "magic numbers" of neutrons and protons, known magic numbers are 2, 8, 20, 28, 50, 82, and 126; however, further numbers are predicted to exist. One element that is predicted to be quite stable is unbihexium, in particular, its isotope unbihexium-310, which would be "doubly magic" (both its atomic number of 126 and neutron number of 184 are thought to be magic) and so would be the most likely to be very stable. The next lightest doubly magic isotope of an element is lead-208, the heaviest stable isotope of any element and the most stable heavy metal.