Proton

A proton is a positively charged subatomic particle found in atomic nuclei. It is a composite particle made of two up quarks and one down quark, bound by the strong interaction.

Up quark: An elementary quark with electric charge +2/3 of the elementary charge. Two up quarks contribute to the proton’s composition and its net positive charge.
Strong interaction: The fundamental interaction that binds quarks into hadrons and holds atomic nuclei together. It confines the proton’s quarks and contributes most of its mass.
Atomic number: The number of protons in an atomic nucleus, defining the chemical element. Changing the proton count changes an atom into a different element.
Neutron: A neutral subatomic particle found in atomic nuclei, composed of two down quarks and one up quark. It shares the proton’s nuclear setting but has no net electric charge.
Down quark: An elementary quark with electric charge −1/3 of the elementary charge. The proton contains one down quark alongside two up quarks.
Proton spin: The intrinsic angular momentum of a proton, equal to one-half in quantum units. Its origin reflects contributions from quark spins, gluons, and their motion.
Hydrogen: The chemical element with one proton in its most common isotope. A hydrogen-1 nucleus is a single proton.
Electron: A negatively charged elementary particle found outside atomic nuclei and in other systems. Its opposite charge balances the proton in electrically neutral atoms.
Quantum chromodynamics: The quantum field theory describing quarks, gluons, and the strong interaction. It describes how gluons bind the proton’s quarks.
Proton radius puzzle: The historical discrepancy between measurements of the proton’s charge radius using different methods. Different radius measurements probe the proton’s electromagnetic structure.

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