A filled shell would have " magic numbers " of neutrons and protons. One possible magic number of neutrons for spherical nuclei is 184, and some possible matching proton numbers are 114, 120 and 126. These configurations imply that the most stable spherical isotopes would be flerovium -298, unbinilium -304 and unbihexium -310. Of particular note is Fl, which would be " doubly magic " (both its proton number of 114 and neutron number of 184 are thought to be magic). This doubly magic configuration is the most likely to have a very long half-life. The next lighter doubly magic spherical nucleus is lead -208, the heaviest known stable nucleus and most stable heavy metal.