 From the late 1940s to the early 1960s , the early days of the synthesis of heavier and heavier transuranium elements , it was predicted that since such heavy elements did not occur naturally , they would have shorter and shorter half @-@ lives to spontaneous fission , until they stopped existing altogether at around element 108 ( now known as hassium ) . Initial work in the synthesis of the actinides appeared to confirm this . The nuclear shell model , introduced in the late 1960s , stated that the protons and neutrons formed shells within a nucleus , somewhat analogous to electrons forming electron shells within an atom . The noble gases are unreactive due to their having full electron shells ; thus it was theorized that elements with full nuclear shells – having so @-@ called " magic " numbers of protons or neutrons – would be stabilized against radioactive decay . A doubly magic isotope , having magic numbers of both protons and neutrons , would be especially stabilized , and it was calculated that the next doubly magic isotope after lead @-@ 208 would be flerovium @-@ 298 with 114 protons and 184 neutrons , which would form the centre of a so @-@ called " island of stability " . This island of stability , supposedly ranging from copernicium ( element 112 ) to ununoctium ( 118 ) , would come after a long " sea of instability " from elements 101 ( mendelevium ) to 111 ( roentgenium ) , and the flerovium isotopes in it were speculated in 1966 to have half @-@ lives in excess of a hundred million years . It took thirty years for the first isotopes of flerovium to be synthesized . More recent work suggests that the local islands of stability around hassium and flerovium are due to these nuclei being respectively deformed and oblate , which make them resistant to spontaneous fission , and that the true island of stability for spherical nuclei occurs at around unbibium @-@ 306 ( with 122 protons and 184 neutrons ) . 
