The bottom region of the surface defined by plotting the numbers of protons and neutrons of a nucleus horizontally and the energy per nucleon of the nucleus vertically. Stable nuclei form a ‘valley’ with the most stable nuclei such as iron and nickel being at the bottom of the valley. The concept of the valley of stability enables several important topics in nuclear physics such as nuclear fission and nuclear fusion to be understood in a clear physical way. There are occasional ‘pits’ in the valley of stability due to nuclei with magic numbers being more stable than nuclei with similar numbers of protons and neutrons.