Nucleons and scale
Protons and neutrons are collectively called nucleons. Protons carry positive electric charge; neutrons have no net charge and a similar mass. Ordinary hydrogen provides the simplest exception to a two-component picture: its nucleus consists of one proton and no neutron.
Identifying a nuclide
Atomic number counts protons, while mass number counts all nucleons. Isotopes share an atomic number but have different neutron numbers. Mass number is an integer count, not the measured atomic mass expressed in atomic mass units, a distinction important when comparing nuclear energies.
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Binding and instability
Short-range nuclear interactions bind nucleons despite electrical repulsion between protons. Stability depends on nuclear composition and structure. An unstable nucleus may undergo radioactive decay, producing particles or electromagnetic radiation; its daughter nucleus may itself remain unstable and decay again.
From scattering to nuclear research
Large deflections of a small fraction of alpha particles passing through thin foil supported Rutherford’s nuclear atom. Subsequent studies distinguished protons, neutrons and nuclear energy states, opening investigations of isotopes and transformations that cannot be explained by rearranging electrons alone.