A proton, represented by p or p⁺, is a subatomic particle with positive electric charge. Together with the neutron, it forms the atomic nucleus of an atom; the nucleus of ordinary hydrogen consists of a single proton. Protons and neutrons are collectively called nucleons. A proton is not elementary: it contains quarks and gluons, interacting through the strong force. The number of protons in a nucleus determines which chemical element it belongs to. (energy.gov)
Physical properties
A proton carries charge +e, equal in magnitude and opposite in sign to the charge of an electron. The elementary charge is exactly 1.602176634 × 10⁻¹⁹ coulomb in the International System of Units. Its rest mass is approximately 1.672621926 × 10⁻²⁷ kilograms, or 1.007276467 unified atomic mass units. Its rest-energy equivalent is about 938.272 megaelectronvolts, and it is approximately 1,836 times as massive as an electron. These numerical values, except for the exact elementary charge, are measured quantities with uncertainties. (tsapps.nist.gov)
The proton has spin quantum number ½ and is therefore a fermion. It also possesses a magnetic moment, allowing its quantum states to respond to an applied magnetic field. Unlike an elementary point particle, it has an extended internal structure. The 2022 CODATA adjustment gives its root-mean-square charge radius as 0.84075 ± 0.00064 femtometres, where one femtometre is 10⁻¹⁵ metre. This radius characterizes its charge distribution rather than a sharply defined solid surface. (goldbook.iupac.org)
Quark structure and the origin of mass
Within the Standard Model, the proton is classified as a baryon, a type of composite particle with three valence quarks. Its valence composition is two up quarks and one down quark, conventionally written uud. Each up quark has charge +⅔e, while the down quark has charge −⅓e, giving a total charge of +e. The neutron’s corresponding composition is udd. (energy.gov)
The quarks interact through the strong interaction, mediated by gluons. Its underlying theory is quantum chromodynamics, or QCD. The three-valence-quark description identifies the proton’s net quark content, but does not exhaust its structure: gluons and sea quark–antiquark contributions are also essential. Consequently, the proton is not accurately pictured as three small objects held together by rigid connectors. Its internal structure is described through quantum fields and distributions of constituents’ momenta. (energy.gov)
The masses of the up and down quarks account for only a small fraction of the proton’s mass. Most arises from the dynamics of the strongly interacting quark and gluon fields. Through mass–energy equivalence, energy associated with these fields contributes to the mass of the composite particle. Thus, the origin of proton mass differs substantially from simply adding the masses acquired by its quarks through their interactions with the Higgs field. (arxiv.org)
Protons in atoms and nuclei
The proton count, called the atomic number and denoted Z, fixes an element’s position in the periodic table. Hydrogen has one proton, carbon six, and oxygen eight. Nuclei belonging to the same element can contain different numbers of neutrons; these are its isotopes. The mass number A is the total number of protons and neutrons, and must be distinguished from the experimentally measured atomic mass. (energy.gov)
Proton charge also establishes the positive charge of a nucleus and its electromagnetic attraction to electrons. In a neutral atom, electron and proton numbers are equal; changing the electron number produces an ion without changing the element. Changing the proton number instead changes the element itself. This distinction separates ordinary chemical ionization from nuclear transformations. (home.web.cern.ch)
Discovery
Ernest Rutherford identified hydrogen nuclei emitted when nitrogen was bombarded with alpha particles. His results, published in 1919, established that the hydrogen nucleus was a constituent of a heavier nucleus. The observation formed an important step toward understanding nuclear transformations and the composition of matter. (cds.cern.ch)
Rutherford introduced the name proton in 1920. The discovery did not immediately produce the modern picture of nuclei or proton structure: those developments depended on subsequent discoveries, including the neutron and the quark structure of nucleons. (cerncourier.com)
Chemical meaning
In acid–base chemistry, “proton” commonly denotes the hydrogen cation transferred from an acid to a base. A Brønsted acid is a species capable of donating such a particle. This chemical usage concerns transfer between molecular species, not the removal of a proton from a heavier atomic nucleus. (goldbook.iupac.org)
For isotope-independent terminology, IUPAC uses hydron for H⁺. Strictly, a bare proton is the nucleus of hydrogen-1; hydrogen cations containing a neutron or two neutrons are instead deuterons or tritons. The general term hydron avoids specifying which hydrogen isotope is involved. (old.goldbook.iupac.org)
Experimental uses and stability
Proton beams are important tools in particle physics. The Large Hadron Collider at CERN is designed to accelerate and collide protons, as well as heavy ions. Such collisions probe matter at high energies and produce particles whose properties can be reconstructed from detector measurements. (home.web.cern.ch)
The magnetic properties of hydrogen nuclei also underpin most magnetic resonance imaging. Magnetic fields and radiofrequency pulses manipulate their quantum states, while the resulting signals provide information about their molecular environments. The protons remain constituents of the sample’s molecules rather than becoming an extracted beam. (nibib.nih.gov)
Free protons are experimentally stable. Searches for hypothetical proton decay, including those conducted by Super-Kamiokande, have found no evidence of it. Such experiments establish lower limits on lifetimes for specified decay channels; they do not prove that the proton’s lifetime is infinite. (www-sk.icrr.u-tokyo.ac.jp)