Nuclear magic number

A proton or neutron count associated with an especially stable, closed shell in an atomic nucleus. The established values are 2, 8, 20, 28, 50, 82, and 126.

Nuclear shell model: A model of atomic nuclei in which protons and neutrons occupy quantized energy levels. Magic numbers emerge when this model's nuclear energy levels form large gaps between filled and unfilled shells.
Helium-4: A helium isotope with two protons and two neutrons, both magic-number counts. Its doubly closed proton and neutron shells make it an exceptionally tightly bound light nucleus.
Doubly magic nucleus: A nucleus with magic numbers of both protons and neutrons. It combines two shell closures, often producing especially strong stability and distinctive structure.
Island of inversion: A region of neutron-rich nuclei where deformed configurations become more favorable than expected closed-shell structures. It shows that the conventional neutron shell closure at 20 can weaken in neutron-rich nuclei.
Nuclear shell closure: The filling of a nuclear shell that leaves a large energy gap before the next available level. A closed shell is the structural condition associated with a magic proton or neutron count.
Oxygen-16: A stable oxygen isotope containing eight protons and eight neutrons. Both nucleon counts are magic, making it a canonical doubly magic nucleus.
Island of stability: A predicted region of superheavy nuclei with longer lifetimes than neighboring nuclides. Proposed shell closures may stabilize superheavy nuclei within this region.
Shell evolution: Changes in nuclear shell gaps as proton and neutron numbers vary. It explains why familiar magic numbers may shift or lose strength far from stable nuclei.
Spin–orbit interaction: An interaction coupling a particle's spin to its orbital motion. Its strong effect on nuclear energy levels explains the observed magic numbers beyond 20.
Calcium-40: A stable calcium isotope containing 20 protons and 20 neutrons. Its two shell closures make it a standard doubly magic nucleus.

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