Stable nucleus
A nucleus that does not undergo radioactive decay and remains unchanged over time. Stability depends on the balance between protons and neutrons (neutron-to-proton ratio) and is related to nuclear binding energy. Light nuclei are typically stable when the neutron-to-proton ratio is approximately 1:1, while heavier nuclei require more neutrons relative to protons to remain stable.
Real World
Iron-56 has the highest binding energy per nucleon of any nucleus, which is why the cores of massive stars like Betelgeuse fuse elements only up to iron before collapsing in a supernova.
Exam Focus
Link stability to binding energy per nucleon — examiners expect you to reference the N–Z curve or binding energy graph.
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