Nucleon

A nucleon is a proton or neutron, the particles that make up an atomic nucleus. Nucleons are composite particles bound by the strong interaction.

Proton: A stable, positively charged baryon containing two up quarks and one down quark. One of the two kinds of nucleons; its positive charge shapes nuclear structure.
Nuclear force: The residual strong interaction that binds protons and neutrons inside atomic nuclei. It holds nucleons together despite the electric repulsion between protons.
Nuclear binding energy: The energy required to separate a nucleus completely into its constituent protons and neutrons. Its variation per nucleon explains why some nuclear reactions release energy.
Electron: A stable elementary particle with negative electric charge, found outside atomic nuclei in ordinary atoms. Unlike nucleons, electrons are leptons and are not constituents of the nucleus.
Neutron: A baryon containing one up quark and two down quarks, with no net electric charge. The other kind of nucleon; its presence lets nuclei contain more protons without added electric repulsion.
Yukawa interaction: A model of a force produced by exchanging massive particles, proposed to describe nuclear binding. Pion exchange gives a useful description of the force between nucleons at moderate distances.
Nuclear fission: The splitting of a heavy atomic nucleus into lighter nuclei, often releasing neutrons and energy. Fission rearranges nucleons into more tightly bound products and can sustain a chain reaction.
Pion: A meson made of a quark and an antiquark, occurring in charged and neutral forms. Pions can mediate nuclear forces but are not the proton-or-neutron constituents of nuclei.
Quark: An elementary particle that combines with other quarks to form hadrons such as protons and neutrons. Up and down quarks are the constituents of ordinary nucleons.
Nuclear shell model: A model in which nucleons occupy quantized energy levels within an atomic nucleus. It explains how individual nucleons fill nuclear orbitals and produce shell closures.

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