Out of about 2500 known nuclides, less than 300 (shown in black) are stable. When plotted on a Segre chart (see below), the stable nuclides are seen to line up along what’s called the valley of stability. Apparently, the N/Z ratio for stable nuclei increases as the nuclei grow in size.
Some insights we can draw from the Segre chart are:
- Lead-208 () is the largest known stable nuclide. Nuclei larger than lead-208 tend to undergo alpha decay (shown in yellow) to quickly shed some “unwanted” nucleons.
- The nuclei above the valley of stability have too many neutrons and too few protons. As such, they tend to undergo beta-minus decay (shown in blue) to swap a neutron for a proton (and an electron).
- The nuclei below the valley of stability have too many protons and too few neutrons. As such, they tend to undergo beta-plus decay (shown in orange) to swap a proton for a neutron (and a positron).