Nuclear stability

Nuclear stability is a nucleus’s resistance to spontaneous transformation into a lower-energy configuration. It depends chiefly on the balance of nuclear forces, proton repulsion, and the numbers of protons and neutrons.

Nuclear binding energy: The energy required to separate a nucleus into its constituent protons and neutrons. Greater binding per nucleon generally makes a nucleus harder to disassemble.
Atomic nucleus: The dense central part of an atom, composed of protons and neutrons. Stability describes whether this central assembly persists or transforms.
Iron-56: A stable iron isotope with 26 protons and 30 neutrons, near the peak of binding energy per nucleon. Its high binding per nucleon illustrates why fusion of light nuclei can release energy.
Valley of stability: The band of stable and long-lived nuclides plotted by proton number against neutron number. It maps how the neutron–proton balance changes across stable nuclei.
Strong nuclear force: The short-range fundamental interaction that binds protons and neutrons inside atomic nuclei. Its attraction counteracts the electrical repulsion between protons.
Proton: A positively charged subatomic particle found in atomic nuclei. Proton number sets nuclear charge and the repulsion stability must overcome.
Lead-208: A stable lead isotope containing 82 protons and 126 neutrons. Its doubly magic proton and neutron counts exemplify shell-enhanced stability.
Nuclear drip line: The boundary beyond which adding a proton or neutron produces a nucleus that promptly emits it. At the drip lines, nuclei cannot retain an extra nucleon even briefly.
Radioactive decay: The spontaneous transformation of an unstable atomic nucleus, often accompanied by emitted particles or radiation. Decay is the process that reveals a nucleus's instability.
Neutron: An electrically neutral subatomic particle found in atomic nuclei. Neutrons add strong-force attraction without adding proton-proton electrical repulsion.

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