The figure at right shows the average binding energy per nucleon across the valley of stability for nuclides with mass number A = 125. At the bottom of this curve is tellurium (52 Te), which is stable. Nuclides to the left of 52 Te are unstable with an excess of neutrons, while those on the right are unstable with an excess of protons. A nuclide on the left therefore undergoes β decay, which converts a neutron to a proton, hence shifts the nuclide to the right and toward greater stability. A nuclide on the right similarly undergoes β decay, which shifts the nuclide to the left and toward greater stability.