Fig 7Which nuclei are stableprotons against neutronsstable line bends away
stable nucleiequal numbers of protons and neutronsEvery stable nucleus sits on the solid line. Light nuclei like carbon have equal numbers of protons and neutrons, but heavier ones need more and more extra neutrons, because every proton repels every other proton while the strong force only reaches its neighbours. Lead-208 has 126 neutrons to 82 protons. Past lead no number of neutrons is enough, and every nucleus is radioactive.