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Quark structure proton.svg
Classification Baryon
Composition 2 up quarks, 1 down quark
Statistics Fermionic
Interactions Gravity, Electromagnetic, Weak, Strong
Symbol p, p+, N+
Antiparticle Antiproton
Theorized William Prout (1815)
Discovered Ernest Rutherford (1917–1919, named by him, 1920)

1.672621777(74)×10−27 kg[1]
938.272046(21) MeV/c2[1]

1.007276466812(90) u[1]
Mean lifetime >2.1×1029 years (stable)
Electric charge +1 e
1.602176565(35)×10−19 C[1]
Charge radius 0.8775(51) fm[1]
Electric dipole moment <5.4×10−24 e·cm
Electric polarizability 1.20(6)×10−3 fm3
Magnetic moment

1.410606743(33)×10−26 J·T−1[1]
1.521032210(12)×10−3 μB[1]

2.792847356(23) μN[1]
Magnetic polarizability 1.9(5)×10−4 fm3
Spin 12
Isospin 12
Parity +1
Condensed I(JP) = 12(12+)

The proton is a subatomic particle with the symbol p or p+ and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom. Protons and neutrons are collectively referred to as "nucleons". The number of protons in the nucleus of an atom is referred to as its atomic number. Since each element has a unique number of protons, each element has its own unique atomic number. The name proton was given to the hydrogen nucleus by Ernest Rutherford in 1920, because in previous years he had discovered that the hydrogen nucleus (known to be the lightest nucleus) could be extracted from the nuclei of nitrogen by collision, and was thus a candidate to be a fundamental particle and building block of nitrogen, and all other heavier atomic nuclei.

In the modern Standard Model of particle physics, the proton is a hadron, and like the neutron, the other nucleon (particle present in atomic nuclei), is composed of three quarks. Prior to that model becoming a consensus in the physics community, the proton was considered a fundamental particle. In the modern view, a proton is composed of three valence quarks: two up quarks and one down quark. The rest masses of the quarks are thought to contribute only about 1% of the proton's mass. The remainder of the proton mass is due to the kinetic energy of the quarks and to the energy of the gluon fields that bind the quarks together.

Because the proton is not a fundamental particle, it possesses a physical size—although this is not perfectly well-defined since the surface of a proton is somewhat fuzzy, due to being defined by the influence of forces that do not come to an abrupt end. The proton is about 1.6–1.7 fm in diameter.[2]


At sufficiently low temperatures, free protons will bind to electrons. However, the character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the electron cloud of an atom. The result is a protonated atom, which is a chemical compound of hydrogen. In vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom, which is chemically a free radical. Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules (H2), which are the most common molecular component of molecular clouds in interstellar space. Such molecules of hydrogen on Earth may then serve (among many other uses) as a convenient source of protons for accelerators (as used in proton therapy) and other hadron particle physics experiments that require protons to accelerate, with the most powerful and noted example being the Large Hadron Collider.


Protons are spin-½ fermions and are composed of three valence quarks,[3] making them baryons (a sub-type of hadrons). The two up quarks and one down quark of the proton are held together by the strong force, mediated by gluons.[2] A modern perspective has the proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs of sea quarks. The proton has an approximately exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm.[4]



The spontaneous decay of free protons has never been observed, and the proton is therefore considered a stable particle. However, some grand unified theories of particle physics predict that proton decay should take place with lifetimes of the order of 1036 years, and experimental searches have established lower bounds on the mean lifetime of the proton for various assumed decay products.

Experiments at the Super-Kamiokande detector in Japan gave lower limits for proton mean lifetime of 6.6×1033 years for decay to an antimuon and a neutral pion, and 8.2×1033 years for decay to a positron and a neutral pion.[5] Another experiment at the Sudbury Neutrino Observatory in Canada searched for gamma rays resulting from residual nuclei resulting from the decay of a proton from oxygen-16. This experiment was designed to detect decay to any product, and established a lower limit to the proton lifetime of 2.1×1029 years.[6]


p+ + en + ν


Quarks and the mass of the proton[edit]

In quantum chromodynamics, the modern theory of the nuclear force, most of the mass of the proton and the neutron is explained by special relativity. The mass of the proton is about 80–100 times greater than the sum of the rest masses of the quarks that make it up, while the gluons have zero rest mass. The extra energy of the quarks and gluons in a region within a proton, as compared to the rest energy of the quarks alone in the QCD vacuum, accounts for almost 99% of the mass. The rest mass of the proton is, thus, the invariant mass of the system of moving quarks and gluons that make up the particle, and, in such systems, even the energy of massless particles is still measured as part of the rest mass of the system.

Two terms are used in referring to the mass of the quarks that make up protons: current quark mass refers to the mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark.[7] These masses typically have very different values. As noted, most of a proton's mass comes from the gluons that bind the constituent quarks together, rather than from the quarks themselves. While gluons are inherently massless, they possess energy—to be more specific, quantum chromodynamics binding energy (QCBE)—and it is this that contributes so greatly to the overall mass of the proton (see mass in special relativity). A proton has a mass of approximately 938 MeV/c2, of which the rest mass of its three valence quarks contributes only about 11 MeV/c2; much of the remainder can be attributed to the gluons' QCBE.[8]

The internal dynamics of the proton are complicated, because they are determined by the quarks' exchanging gluons, and interacting with various vacuum condensates. Lattice QCD provides a way of calculating the mass of the proton directly from the theory to any accuracy, in principle. The most recent calculations[9][10] claim that the mass is determined to better than 4% accuracy, even to 1% accuracy (see Figure S5 in Dürr et al.[10]). These claims are still controversial, because the calculations cannot yet be done with quarks as light as they are in the real world. This means that the predictions are found by a process of extrapolation, which can introduce systematic errors.[11] It is hard to tell whether these errors are controlled properly, because the quantities that are compared to experiment are the masses of the hadrons, which are known in advance.

These recent calculations are performed by massive supercomputers, and, as noted by Boffi and Pasquini: "a detailed description of the nucleon structure is still missing because ... long-distance behavior requires a nonperturbative and/or numerical treatment..."[12] More conceptual approaches to the structure of the proton are: the topological soliton approach originally due to Tony Skyrme and the more accurate AdS/QCD approach that extends it to include a string theory of gluons, various QCD-inspired models like the bag model and the constituent quark model, which were popular in the 1980s, and the SVZ sum rules, which allow for rough approximate mass calculations. These methods do not have the same accuracy as the more brute-force lattice QCD methods, at least not yet.

Charge radius[edit]

The internationally-accepted value of the proton's charge radius is 0.8768 fm (see orders of magnitude for comparison to other sizes). This value is based on measurements involving a proton and an electron.

However, since 5 July 2010, an international research team has been able to make measurements involving an exotic atom made of a proton and a negatively-charged muon. After a long and careful analysis of those measurements, the team concluded that the root-mean-square charge radius of a proton is "0.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value of 0.8768(69) fm".[13] In January 2013, an updated value for the charge radius of a proton—0.84087(39) fm—was published. The precision was improved by 1.7 times, but the difference with CODATA value persisted at 7σ significance.[14]

The international research team that obtained this result at the Paul Scherrer Institut (PSI) in Villigen (Switzerland) includes scientists from the Max Planck Institute of Quantum Optics (MPQ) in Garching, the Ludwig-Maximilians-Universität (LMU) Munich and the Institut für Strahlwerkzeuge (IFWS) of the Universität Stuttgart (both from Germany), and the University of Coimbra, Portugal.[15][16] They are now attempting to explain the discrepancy, and re-examining the results of both previous high-precision measurements and complicated calculations. If no errors are found in the measurements or calculations, it could be necessary to re-examine the world's most precise and best-tested fundamental theory: quantum electrodynamics.[17]

Interaction of free protons with ordinary matter[edit]

Although protons have affinity for oppositely-charged electrons, free protons must lose sufficient velocity (and kinetic energy) in order to become closely associated and bound to electrons, since this is a relatively low-energy interaction. High energy protons, in traversing ordinary matter, lose energy by collisions with atomic nuclei, and by ionization of atoms (removing electrons) until they are slowed sufficiently to be captured by the electron cloud in a normal atom.


Proton in chemistry[edit]

Atomic number[edit]


with 35 − 17 = 18 neutrons and 37
with 37 − 17 = 20 neutrons.

Hydrogen ion[edit]



The transfer of H+


Proton nuclear magnetic resonance (NMR)[edit]




Ernest Rutherford at the first Solvay Conference, 1911




Rutherford knew hydrogen to be the simplest and lightest element and was influenced by Prout's hypothesis that hydrogen was the building block of all elements. Discovery that the hydrogen nucleus is present in all other nuclei as an elementary particle, led Rutherford to give the hydrogen nucleus a special name as a particle, since he suspected that hydrogen, the lightest element, contained only one of these particles. He named this new fundamental building block of the nucleus the proton, after the neuter singular of the Greek word for "first", πρῶτον. However, Rutherford also had in mind the word protyle as used by Prout. Rutherford spoke at the British Association for the Advancement of Science at its Cardiff meeting beginning 24 August 1920.[20] Rutherford was asked by Oliver Lodge for a new name for the positive hydrogen nucleus to avoid confusion with the neutral hydrogen atom. He initially suggested both proton and prouton (after Prout).[21] Rutherford later reported that the meeting had accepted his suggestion that the hydrogen nucleus be named the "proton", following Prout's word "protyle".[22] The first use of the word "proton" in the scientific literature appeared in 1920.[23]




Protons also occur in from extrasolar origin in space, from galactic cosmic rays, where they make up about 90% of the total particle flux. These protons often have higher energy than solar wind protons, but their intensity is far more uniform and less variable than protons coming from the Sun, the production of which is heavily affected by solar proton events such as coronal mass ejections.

Research has been performed on the dose-rate effects of protons, as typically found in space travel, on human health.[25][26] To be more specific, there are hopes to identify what specific chromosomes are damaged, and to define the damage, during cancer development from proton exposure.[25] Another study looks into determining "the effects of exposure to proton irradiation on neurochemical and behavioral endpoints, including dopaminergic functioning, amphetamine-induced conditioned taste aversion learning, and spatial learning and memory as measured by the Morris water maze.[26] Electrical charging of a spacecraft due to interplanetary proton bombardment has also been proposed for study.[27] There are many more studies that pertain to space travel, including galactic cosmic rays and their possible health effects, and solar proton event exposure.




See also[edit]


  2. ^ a b W.N. Cottingham, D.A. Greenwood (1986). An Introduction to Nuclear Physics. Cambridge University Press. p. 19. 
  3. ^ R.K. Adair (1989). The Great Design: Particles, Fields, and Creation. Oxford University Press. p. 214. 
  5. ^ H. Nishino et al. (Kamiokande collaboration) (2009). "Search for Proton Decay via pe+ π0 and pμ+ π0 in a Large Water Cherenkov Detector". Physical Review Letters 102 (14): 141801. arXiv:0903.0676. Bibcode:2009PhRvL.102n1801N. doi:10.1103/PhysRevLett.102.141801. 
  6. ^ S.N. Ahmed et al. (SNO Collaboration) (2004). "Constraints on nucleon decay via invisible modes from the Sudbury Neutrino Observatory". Physical Review Letters 92 (10): 102004. arXiv:hep-ex/0310030. Bibcode:2004PhRvL..92j2004A. doi:10.1103/PhysRevLett.92.102004. PMID 15089201. 
  9. ^ See this news report and links
  10. ^ a b S. Dürr, Z. Fodor, J. Frison, C. Hoelbling, R. Hoffmann, S. D. Katz, S. Krieg, T. Kurth, L. Lellouch, T. Lippert, K. K. Szabo, and G. Vulvert (21 November 2008). "Ab Initio Determination of Light Hadron Masses". Science 322 (5905): 1224–7. arXiv:0906.3599. Bibcode:2008Sci...322.1224D. doi:10.1126/science.1163233. PMID 19023076. 
  11. ^ C. F. Perdrisat, V. Punjabi, M. Vanderhaeghen (2007). "Nucleon Electromagnetic Form Factors". Prog Part Nucl Phys 59 (2): 694–764. arXiv:hep-ph/0612014. Bibcode:2007PrPNP..59..694P. doi:10.1016/j.ppnp.2007.05.001. 
  12. ^ Sigfrido Boffi & Barbara Pasquini (2007). "Generalized parton distributions and the structure of the nucleon". Rivista del Nuovo Cimento 30. arXiv:0711.2625. Bibcode:2007NCimR..30..387B. doi:10.1393/ncr/i2007-10025-7. 
  13. ^ Randolf Pohl, Aldo Antognini, François Nez, Fernando D. Amaro, François Biraben, João M. R. Cardoso, Daniel S. Covita, Andreas Dax, Satish Dhawan, Luis M. P. Fernandes, Adolf Giesen, Thomas Graf, Theodor W. Hänsch, Paul Indelicato, Lucile Julien, Cheng-Yang Kao, Paul Knowles, Eric-Olivier Le Bigot, Yi-Wei Liu, José A. M. Lopes, Livia Ludhova, Cristina M. B. Monteiro, Françoise Mulhauser, Tobias Nebel, Paul Rabinowitz, et al. (8 July 2010). "The size of the proton". Nature 466 (7303): 213–216. Bibcode:2010Natur.466..213P. doi:10.1038/nature09250. PMID 20613837. Retrieved 2010-07-09. 
  15. ^ New proton measurements may throw physics a curve
  17. ^ Researchers Observes Unexpectedly Small Proton Radius in a Precision Experiment
  19. ^ R.H. Petrucci, W.S. Harwood, and F.G. Herring (2002). General Chemistry (8th ed.). p. 41. 
  20. ^ See meeting report and announcement
  21. ^ Romer A (1997). "Proton or prouton? Rutherford and the depths of the atom". Amer. J. Phys. 65 (8): 707. Bibcode:1997AmJPh..65..707R. doi:10.1119/1.18640. 
  22. ^ Rutherford reported acceptance by the British Association in a footnote to a 1921 paper by O. Masson in the Philosophical Magazine (O. Masson, Phil. Mag. 41, 281, 1921)
  23. ^ Pais, Inward Bound, first edition, Oxford Press, 1986, page 296. Pais reported that he believed the first science literature use of the word proton occurs in the article Nature, 106: 357, 1920.
  26. ^ a b B. Shukitt-Hale, A. Szprengiel, J. Pluhar, B.M. Rabin, and J.A. Joseph. "The effects of proton exposure on neurochemistry and behavior". Elsevier/COSPAR. Retrieved 2009-06-12. 
  27. ^ N.W. Green and A.R. Frederickson. "A Study of Spacecraft Charging due to Exposure to Interplanetary Protons". Jet Propulsion Laboratory. Retrieved 2009-06-12. 
  28. ^ H. Planel (2004). Space and life: an introduction to space biology and medicine. CRC Press. pp. 135–138. ISBN 0-415-31759-2. 
  29. ^ G. Gabrielse (2006). "Antiproton mass measurements". International Journal of Mass Spectrometry 251 (2–3): 273–280. Bibcode:2006IJMSp.251..273G. doi:10.1016/j.ijms.2006.02.013. 

External links[edit]