# Proton
The proton is a stable subatomic particle with electric charge +1, spin 1/2, and a rest mass of 1.007 276 470(12) u (about 938 MeV/c²), which is found in the nuclei of atoms and forms the nucleus of ordinary hydrogen on its own.[1](https://goldbook.iupac.org/terms/view/P04906) • [2](https://opendata.cern.ch/glossary/Proton) • [3](https://pubchem.ncbi.nlm.nih.gov/compound/Proton) IUPAC classifies it both as a hydron, meaning a hydrogen ion, and as a nucleon.[1](https://goldbook.iupac.org/terms/view/P04906) It is a composite hadron made of two up quarks and one down quark held together by gluons, and it is about 1836 times as massive as the electron.[2](https://opendata.cern.ch/glossary/Proton) • [4](https://www.encyclopedia.com/science-and-technology/physics/physics/proton)
| Key fact | Detail |
|---|---|
| Charge and spin | Charge number +1; spin quantum number 1/2[1](https://goldbook.iupac.org/terms/view/P04906) |
| Rest mass | 1.007 276 470(12) u, about 938 MeV/c², roughly 1836 times the electron mass[1](https://goldbook.iupac.org/terms/view/P04906) • [2](https://opendata.cern.ch/glossary/Proton) |
| Composition | Two up quarks and one down quark held together by gluons[2](https://opendata.cern.ch/glossary/Proton) • [4](https://www.encyclopedia.com/science-and-technology/physics/physics/proton) |
| Magnetic moment | 2.793 nuclear magnetons[5](https://snl.no/proton) |
| Stability | Free protons do not decay on their own; experimental searches place the lifetime beyond roughly 10³³ years[6](https://www.energy.gov/science/doe-explainsprotons) • [7](https://www.hyperphysics.gsu.edu/hbase/Particles/proton.html) |
| Charge radius | 0.84087(39) fm from muonic-hydrogen spectroscopy (2013), a value with which electron scattering and ordinary hydrogen spectroscopy later converged[8](https://pubmed.ncbi.nlm.nih.gov/23349284/) • [9](https://www.nature.com/articles/s42254-020-0229-x) |
| Role in matter | The number of protons fixes an element's atomic number; hydrogen's nucleus is a single proton with no neutrons[6](https://www.energy.gov/science/doe-explainsprotons) |
## Discovery and naming
**From nucleus to particle.** In 1909 the New Zealand-British physicist [Ernest Rutherford](https://www.edgepedia.org/ernest-rutherford) instructed [Hans Geiger](https://www.edgepedia.org/hans-geiger) and Ernst Marsden, then an undergraduate, to perform the gold foil experiment, in which alpha particles were fired at thin gold foil. The results indicated that the atom contains a positively charged nucleus. In 1919 Rutherford identified the proton as the fundamental unit of positive electrical charge in the atom.[4](https://www.encyclopedia.com/science-and-technology/physics/physics/proton)
The name itself was settled around 1920 at an informal meeting of the British Association in [Cardiff](https://www.edgepedia.org/cardiff). The alternative "baron", from the Greek βαρος (weight), had also been considered. Rutherford, who suggested "proton", reported that it met with general approval, partly because it resonated with "protyle", a term for primordial matter invented nearly a century earlier by the British chemist William Prout. In 1921 Orme Masson proposed in the *Philosophical Magazine* that hydrogen nuclei be called "barons", with a footnote from Rutherford noting that the name had already been chosen.[10](https://www.nature.com/articles/s41567-021-01484-w)
## Properties and measured values
The proton's rest mass is 1.007 276 470(12) u according to IUPAC, equivalent to about 1.67262 × 10⁻²⁷ kg, and it is slightly lighter than the neutron.[1](https://goldbook.iupac.org/terms/view/P04906) • [5](https://snl.no/proton) Its magnetic moment is 2.793 nuclear magnetons, and its spin is 1/2 ħ.[5](https://snl.no/proton) Because a bare proton is the nucleus of the light hydrogen atom, it is also the hydrogen ion in chemical contexts.[3](https://pubchem.ncbi.nlm.nih.gov/compound/Proton)
## Internal structure: quarks, gluons, and QCD
According to the theory of the strong interaction, quantum chromodynamics (QCD), the proton is a bound state of three valence quarks, two up quarks and one down quark, held together by gluons. The up quarks each carry +2/3 of the elementary charge and the down quark carries −1/3, summing to the proton's net +1.[2](https://opendata.cern.ch/glossary/Proton) • [4](https://www.encyclopedia.com/science-and-technology/physics/physics/proton) Neutrons contain the complementary set, one up and two down quarks. In addition to the valence quarks, sea quarks, quark-antiquark pairs, constantly pop in and out of existence inside the proton; such pairs can be produced and annihilated without changing the proton's net external appearance, consistent with quark confinement.[6](https://www.energy.gov/science/doe-explainsprotons) • [7](https://www.hyperphysics.gsu.edu/hbase/Particles/proton.html)
**The spin puzzle.** Measurements reported by the European Muon Collaboration at CERN in 1987 showed that the individual quarks contribute only a small fraction of the proton's total spin, a result known as the proton spin puzzle.[5](https://snl.no/proton)
## Stability and decay
Unlike the neutron, the free proton is stable: it does not break down, or decay, on its own, and it is a composite subatomic particle made of smaller particles.[6](https://www.energy.gov/science/doe-explainsprotons) Grand unification theories predict that the proton should nevertheless decay. The simplest theories of this type give a mean lifetime above 10³¹ years, which would imply less than one decay per year in a cubic meter of water, while other models suggest a half-life on the order of 10³² years.[5](https://snl.no/proton) • [7](https://www.hyperphysics.gsu.edu/hbase/Particles/proton.html)
Experiments have searched for such decays without success. The Soudan 2 proton decay experiment ran from 1989 to 2001 without observing any convincing proton decays, and searches have also been carried out at [Super-Kamiokande](https://www.edgepedia.org/super-kamiokande) in Japan. HyperPhysics reports that the proton lifetime has been pushed out to about 10³³ years; the simplest grand-unified lifetime bound of more than 10³¹ years from Store norske leksikon is a theoretical prediction rather than an experimental limit, so the two figures describe different quantities and are not directly comparable.[7](https://www.hyperphysics.gsu.edu/hbase/Particles/proton.html) • [5](https://snl.no/proton)
## Comparison with the neutron and the antiproton
The neutron is about 0.2% more massive than the proton, an energy difference of 1.29 MeV. A [free neutron decays](https://www.edgepedia.org/free-neutron-decay) with a half-life of about 10.3 minutes, but it can be stable inside some nuclei; the proton, by contrast, is stable when free, although a bound proton can convert into a neutron through beta-plus decay or electron capture in suitable nuclei.[7](https://www.hyperphysics.gsu.edu/hbase/Particles/proton.html) • [11](https://www.nndc.bnl.gov/nudat3/guide/betadecay.html) The two particles also differ in quark content, one up and two down quarks for the neutron against two up and one down for the proton.[6](https://www.energy.gov/science/doe-explainsprotons)
The proton's antiparticle, the antiproton, was first discovered experimentally in 1955 at the Bevatron accelerator in California, and antiprotons also occur naturally in cosmic radiation.[5](https://snl.no/proton)
## The proton radius puzzle
The proton's charge radius, a measure of the spatial extent of its charge distribution, has been measured since the 1950s using two techniques: elastic electron-proton scattering and high-resolution spectroscopy of ordinary hydrogen.[9](https://www.nature.com/articles/s42254-020-0229-x) In 2010 a highly precise measurement using, for the first time, spectroscopy of muonic hydrogen, hydrogen in which the orbiting electron is replaced by a muon, disagreed with the previously accepted value, creating what became known as the proton radius puzzle.[9](https://www.nature.com/articles/s42254-020-0229-x)
The 2S(1/2)(F=0)-2P(3/2)(F=1) transition in muonic hydrogen was measured by laser spectroscopy at 54611.16(1.05) GHz, giving r(E) = 0.84087(39) fm for the charge radius; this is roughly ten times as precise as the 2010 CODATA figure but differs from it by 7σ, deepening the puzzle.[8](https://pubmed.ncbi.nlm.nih.gov/23349284/) The same measurements gave a Zemach radius r(Z) = 1.082(37) fm and a magnetic radius r(M) = 0.87(6) fm.[8](https://pubmed.ncbi.nlm.nih.gov/23349284/)
**Resolution.** Subsequently, two measurements, one from electron scattering and one from ordinary hydrogen spectroscopy, were found to agree with the muonic-atom results, suggesting that the puzzle is resolved in favor of the smaller radius.[9](https://www.nature.com/articles/s42254-020-0229-x)
## Applications and practical role
Protons are the workhorse particles of high-energy physics: they are one of the species collided at the [Large Hadron Collider](https://www.edgepedia.org/large-hadron-collider), and proton-proton collisions at the LHC led to the observation of the [Higgs boson](https://www.edgepedia.org/higgs-boson) in 2012.[2](https://opendata.cern.ch/glossary/Proton) • [5](https://snl.no/proton) In medicine, an estimated more than 100,000 patients per year worldwide receive proton therapy, which can deliver radiation more focally and may be gentler on healthy tissue than conventional radiotherapy.[5](https://snl.no/proton)
In chemistry and metrology, the proton is the hydrogen ion, H⁺, and the value 1.007 276 470(12) u is the rest mass of the proton, the nucleus of the light hydrogen atom.[3](https://pubchem.ncbi.nlm.nih.gov/compound/Proton) • [1](https://goldbook.iupac.org/terms/view/P04906) Because the number of protons in a nucleus determines the atomic number, protons also define what element an atom is.[6](https://www.energy.gov/science/doe-explainsprotons)
## References
1. [IUPAC Gold Book – proton (P04906)](https://goldbook.iupac.org/terms/view/P04906)
2. [Proton – CERN Open Data Portal](https://opendata.cern.ch/glossary/Proton)
3. [Proton | H+ | CID 5460653 – PubChem, NIH](https://pubchem.ncbi.nlm.nih.gov/compound/Proton)
4. [Proton – Encyclopedia.com](https://www.encyclopedia.com/science-and-technology/physics/physics/proton)
5. [proton – Store norske leksikon](https://snl.no/proton)
6. [DOE Explains...Protons – U.S. Department of Energy](https://www.energy.gov/science/doe-explainsprotons)
7. [Protons and neutrons – HyperPhysics, Georgia State University](https://www.hyperphysics.gsu.edu/hbase/Particles/proton.html)
8. [Pohl et al. (2013). Proton structure from the measurement of 2S-2P transition frequencies of muonic hydrogen. Nature.](https://pubmed.ncbi.nlm.nih.gov/23349284/)
9. [The proton size – Nature Reviews Physics](https://www.nature.com/articles/s42254-020-0229-x)
10. [Pinning down the proton – Nature Physics](https://www.nature.com/articles/s41567-021-01484-w)
11. [Beta Decay](https://www.nndc.bnl.gov/nudat3/guide/betadecay.html)
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*Topic: Encyclopedia › Physical world and mathematics › Physics › Particles and nuclei › Particle physics › Hadrons and hadron spectroscopy › Nucleons and light baryons*
*Initially written Sep 17, 2026 · Reviewed: Oct 25, 2026 · Edited: Oct 15, 2026; Oct 25, 2026 · Last review: Oct 25, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license