|Nucleus map in 1940|
Map showing the nuclei known before 1940. The stable or quasi-stable nuclei follow a line which stretches along the bottom of the valley of stability. The line ends at the heaviest natural nucleus, uranium 238 a radioactive isotope comprised of 92 protons and 146 neutrons. The area in grey represents those nuclei known today, which are far greater in number but are unstable.