Wikipedia

Proton

For other uses, seeProton (disambiguation).
"P+" redirects here. For the record label, seeJean Dawson.

Aprotonis a stablesubatomic particle,symbol
p
,H+,or1H+with a positiveelectric chargeof +1e(elementary charge). Its mass is slightly less than the mass of aneutronand approximately1836times the mass of anelectron(theproton-to-electron mass ratio). Protons and neutrons, each with a mass of approximately oneatomic mass unit,are jointly referred to asnucleons(particles present in atomic nuclei).

Proton
Thevalence quarkcontent of a proton. Thecolor assignmentof individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated bygluons.
ClassificationBaryon
Composition2up quarks(u), 1down quark(d)
StatisticsFermionic
FamilyHadron
InteractionsGravity,electromagnetic,weak,strong
Symbol
p
,
p+
,
N+
,1
1H+
AntiparticleAntiproton
TheorizedWilliam Prout(1815)
DiscoveredObserved as H+byEugen Goldstein(1886). Identified in other nuclei (and named) byErnest Rutherford(1917–1920).
Mass1.67262192595(52)×10−27kg‍[1]
1.0072764665789(83) Da‍[2]
938.27208943(29)MeV/c2‍[3]
Mean lifetime>3.6×1029years[4](stable)
Electric charge+1e
Charge radius0.8414(19)fm[5]
Electric dipole moment<2.1×10−25e⋅cm[6]
Electric polarizability0.00112(4) fm3
Magnetic moment1.41060679545(60)×10−26J⋅T−1‍[7]
0.00152103220230(45)μB‍[8]
2.79284734463(82)μN‍[9]
Magnetic polarizability1.9(5)×10−4fm3
Spin⁠1/2⁠ħ
Isospin⁠1/2⁠
Parity+1
CondensedI(JP) =⁠1/2⁠(⁠1/2⁠+)

One or more protons are present in thenucleusof everyatom.They provide the attractive electrostatic central force which binds the atomic electrons. The number of protons in the nucleus is the defining property of an element, and is referred to as theatomic number(represented by the symbolZ). Since eachelementis identified by the number of protons in its nucleus, each element has its own atomic number, which determines the number of atomic electrons and consequently the chemical characteristics of the element.

The wordprotonisGreekfor "first", and the name was given to the hydrogen nucleus byErnest Rutherfordin 1920. In previous years, Rutherford had discovered that thehydrogennucleus (known to be the lightest nucleus) could be extracted from the nuclei ofnitrogenby atomic collisions.[10]Protons were therefore a candidate to be a fundamental orelementary particle,and hence a building block of nitrogen and all other heavier atomic nuclei.

Although protons were originally considered to be elementary particles, in the modernStandard Modelofparticle physics,protons are known to be composite particles, containing threevalence quarks,and together withneutronsare now classified ashadrons.Protons are composed of twoup quarksof charge +⁠2/3⁠eeach, and onedown quarkof charge −⁠1/3⁠e.Therest massesof quarks contribute only about 1% of a proton's mass.[11]The remainder of a proton's mass is due toquantum chromodynamics binding energy,which includes thekinetic energyof the quarks and the energy of thegluonfields that bind the quarks together. Theroot mean squarecharge radiusof a proton is about 0.84–0.87fm(1 fm=10−15m).[12][13]In 2019, two different studies, using different techniques, found this radius to be 0.833 fm, with an uncertainty of ±0.010 fm.[14][15]

Free protons occur occasionally on Earth:thunderstormscan produce protons with energies of up to several tens ofMeV.[16][17]At sufficiently low temperatures and kinetic energies, free protons will bind toelectrons.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 theelectron cloudof an atom. The result is a diatomic orpolyatomic ioncontaining hydrogen. In a vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutralhydrogen atom,which is chemically afree 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 ofmolecular cloudsininterstellar space.[18]

Free protons are routinely used for accelerators forproton therapyor various particle physics experiments, with the most powerful example being theLarge Hadron Collider.

Description

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Unsolved problem in physics:
How do the quarks and gluons carry the spin of protons?
(more unsolved problems in physics)

Protons arespin-⁠1/2⁠fermionsand are composed of three valence quarks,[19]making thembaryons(a sub-type ofhadrons). The twoup quarksand onedown quarkof a proton are held together by thestrong force,mediated bygluons.[20]: 21–22 A modern perspective has a proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs ofsea quarks.Protons have a positive charge distribution, which decays approximately exponentially, with a root mean squarecharge radiusof about 0.8 fm.[21]

Protons andneutronsare bothnucleons,which may be bound together by thenuclear forceto formatomic nuclei.The nucleus of the most commonisotopeof thehydrogen atom(with thechemical symbol"H" ) is a lone proton. The nuclei of the heavy hydrogen isotopesdeuteriumandtritiumcontain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.

History

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The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815,William Proutproposed that all atoms are composed of hydrogen atoms (which he called "protyles" ), based on a simplistic interpretation of early values ofatomic weights(seeProut's hypothesis), which was disproved when more accurate values were measured.[22]: 39–42 

Ernest Rutherfordat the firstSolvay Conference,1911
Proton detected in anisopropanolcloud chamber

In 1886,Eugen Goldsteindiscoveredcanal rays(also known as anode rays) and showed that they were positively charged particles (ions) produced from gases. However, since particles from different gases had different values ofcharge-to-mass ratio(q/m), they could not be identified with a single particle, unlike the negativeelectronsdiscovered byJ. J. Thomson.Wilhelm Wienin 1898 identified the hydrogen ion as the particle with the highest charge-to-mass ratio in ionized gases.[23]

Following the discovery of the atomic nucleus byErnest Rutherfordin 1911,Antonius van den Broekproposed that the place of each element in theperiodic table(its atomic number) is equal to its nuclear charge. This was confirmed experimentally byHenry Moseleyin 1913 usingX-ray spectra(More details inAtomic numberunder Moseley's 1913 experiment).

In 1917, Rutherford performed experiments (reported in 1919 and 1925) which proved that the hydrogen nucleus is present in other nuclei, a result usually described as the discovery of protons.[24]These experiments began after Rutherford observed that whenAlpha particleswould strike air, Rutherford could detect scintillation on azinc sulfidescreen produced at a distance well beyond the distance of Alpha -particle range of travel but instead corresponding to the range of travel of hydrogen atoms (protons).[25]After experimentation, Rutherford traced the reaction to the nitrogen in air and found that when Alpha particles were introduced into pure nitrogen gas, the effect was larger. In 1919, Rutherford assumed that the Alpha particle merely knocked a proton out of nitrogen, turning it into carbon. After observing Blackett's cloud chamber images in 1925, Rutherford realized that the Alpha particle was absorbed. If the Alpha particle were not absorbed, then it would knock a proton off of nitrogen creating 3 charged particles (a negatively charged carbon, a proton, and an Alpha particle). It can be shown[26]that the 3 charged particles would create three tracks in the cloud chamber, but instead only 2 tracks in the cloud chamber were observed. The Alpha particle is absorbed by the nitrogen atom. After capture of the Alpha particle, a hydrogen nucleus is ejected, creating a net result of 2 charged particles (a proton and a positively charged oxygen) which make 2 tracks in the cloud chamber. Heavy oxygen (17O), not carbon or fluorine, is the product. This was the first reportednuclear reaction,14N + α →17O + p.Rutherford at first thought of our modern "p" in this equation as a hydrogen ion,H+.

Depending on one's perspective, either 1919 (when it was seen experimentally as derived from another source than hydrogen) or 1920 (when it was recognized and proposed as an elementary particle) may be regarded as the moment when the proton was 'discovered'.

Rutherford knew hydrogen to be the simplest and lightest element and was influenced byProut's hypothesisthat hydrogen was the building block of all elements. Discovery that the hydrogen nucleus is present in other nuclei as an elementary particle led Rutherford to give the hydrogen nucleusH+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 theproton,after the neuter singular of the Greek word for "first",πρῶτον.However, Rutherford also had in mind the wordprotyleas used by Prout. Rutherford spoke at theBritish Association for the Advancement of Scienceat itsCardiffmeeting beginning 24 August 1920.[27]At the meeting, he was asked byOliver Lodgefor a new name for the positive hydrogen nucleus to avoid confusion with the neutral hydrogen atom. He initially suggested bothprotonandprouton(after Prout).[28]Rutherford later reported that the meeting had accepted his suggestion that the hydrogen nucleus be named the "proton", following Prout's word "protyle".[29]The first use of the word "proton" in the scientific literature appeared in 1920.[30][31]

Occurrence

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One or more bound protons are present in the nucleus of every atom. Free protons are found naturally in a number of situations in which energies or temperatures are high enough to separate them from electrons, for which they have some affinity. Free protons exist inplasmasin which temperatures are too high to allow them to combine withelectrons.[32]Free protons of high energy and velocity make up 90% ofcosmic rays,which propagate through theinterstellar medium.[33]Free protons areemitted directlyfromatomic nucleiin some rare types ofradioactive decay.[34]Protons also result (along with electrons andantineutrinos) from theradioactive decay of free neutrons,which are unstable.[35]

Stability

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Main article:Proton decay
Unsolved problem in physics:
Are protons fundamentally stable? Or do they decay with a finite lifetime as predicted by some extensions to the standard model?
(more unsolved problems in physics)

The spontaneous decay of free protons has never been observed, and protons are therefore considered stable particles according to the Standard Model. However, somegrand unified theories(GUTs) of particle physics predict thatproton decayshould take place with lifetimes between 1031and 1036years. Experimental searches have established lower bounds on themean lifetimeof a proton for various assumed decay products.[36][37][38]

Experiments at theSuper-Kamiokandedetector in Japan gave lower limits for protonmean lifetimeof6.6×1033yearsfor decay to anantimuonand a neutralpion,and8.2×1033yearsfor decay to apositronand a neutral pion.[39] Another experiment at theSudbury Neutrino Observatoryin Canada searched forgamma raysresulting 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 a proton lifetime of2.1×1029years.[40]

However, protons are known to transform intoneutronsthrough the process ofelectron capture(also calledinverse beta decay). For free protons, this process does not occur spontaneously but only when energy is supplied. The equation is:


p+
+
e−
→
n
+
ν
e

The process is reversible; neutrons can convert back to protons throughbeta decay,a common form ofradioactive decay.In fact, afree neutrondecays this way, with amean lifetimeof about 15 minutes. A proton can also transform into a neutron throughbeta plus decay(β+ decay).

According toquantum field theory,the mean proper lifetime of protonsbecomes finite when they are accelerating withproper acceleration,anddecreases with increasing.Acceleration gives rise to anon-vanishing probabilityfor the transition
p+
→
n
+
e+
+
ν
e.This was a matter of concern in the later 1990s becauseis a scalar that can be measured by the inertial andcoaccelerated observers.In theinertial frame,the accelerating proton should decay according to the formula above. However, according to the coaccelerated observer the proton is at rest and hence should not decay. This puzzle is solved by realizing that in the coaccelerated frame there is a thermal bath due toFulling–Davies–Unruh effect,an intrinsic effect of quantum field theory. In this thermal bath, experienced by the proton, there are electrons and antineutrinos with which the proton may interact according to the processes:


  1. p+
    +
    e−
    →
    n
    +
    ν
    ,

  2. p+
    +
    ν
    →
    n
    +
    e+
    and

  3. p+
    +
    e−
    +
    ν
    →
    n
    .

Adding the contributions of each of these processes, one should obtain.[41][42][43][44]

Quarks and the mass of a proton

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Inquantum chromodynamics,the modern theory of the nuclear force, most of the mass of protons andneutronsis explained byspecial relativity.The mass of a proton is about 80–100 times greater than the sum of the rest masses of its three valencequarks,while thegluonshave zero rest mass. The extra energy of thequarksandgluonsin a proton, as compared to the rest energy of the quarks alone in theQCD vacuum,accounts for almost 99% of the proton's mass. The rest mass of a proton is, thus, theinvariant massof the system of moving quarks and gluons that make up the particle, and, in such systems, even the energy of massless particles confined to a system isstill measuredas 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 quarkmassrefers to the mass of a quark by itself, whileconstituent quarkmassrefers to the current quark mass plus the mass of thegluonparticle fieldsurrounding the quark.[45]: 285–286 [46]: 150–151 These masses typically have very different values. The kinetic energy of the quarks that is a consequence of confinement is a contribution (seeMass in special relativity). Usinglattice QCDcalculations, the contributions to the mass of the proton are the quark condensate (~9%, comprising the up and down quarks and a sea of virtual strange quarks), the quark kinetic energy (~32%), the gluon kinetic energy (~37%), and the anomalous gluonic contribution (~23%, comprising contributions from condensates of all quark flavors).[47]

The constituent quark model wavefunction for the proton is

The internal dynamics of protons are complicated, because they are determined by the quarks' exchanging gluons, and interacting with various vacuum condensates.Lattice QCDprovides a way of calculating the mass of a proton directly from the theory to any accuracy, in principle. The most recent calculations[48][49]claim that the mass is determined to better than 4% accuracy, even to 1% accuracy (see Figure S5 in Dürret al.[49]). 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 ofextrapolation,which can introduce systematic errors.[50]It is hard to tell whether these errors are controlled properly, because the quantities that are compared to experiment are the masses of thehadrons,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..."[51] More conceptual approaches to the structure of protons are: thetopological solitonapproach originally due toTony Skyrmeand the more accurateAdS/QCD approachthat extends it to include astring theoryof gluons,[52]various QCD-inspired models like thebag modeland theconstituent quarkmodel, which were popular in the 1980s, and theSVZ sum rules,which allow for rough approximate mass calculations.[53]These methods do not have the same accuracy as the more brute-force lattice QCD methods, at least not yet.

Charge radius

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Main article:Proton radius puzzle

TheCODATArecommended value of a proton'scharge radiusis8.4075(64)×10−16m.‍[54]The radius of the proton is defined by a formula that can be calculated byquantum electrodynamicsand be derived from either atomic spectroscopy or by electron–proton scattering. The formula involves a form-factor related to the two-dimensionalpartondiameter of the proton.[55]

A value from before 2010 is based on scattering electrons from protons followed by complex calculation involving scattering cross section based onRosenbluthequation formomentum-transfer cross section), and based on studies of the atomicenergy levelsof hydrogen and deuterium. In 2010 an international research team published a proton charge radius measurement via theLamb shiftin muonic hydrogen (anexotic atommade of a proton and a negatively chargedmuon). As a muon is 200 times heavier than an electron, resulting in a smalleratomic orbital,it is much more sensitive to the proton's charge radius and thus allows a more precise measurement.[56]Subsequent improved scattering and electron-spectroscopy measurements agree with the new small radius. Work continues to refine and check this new value.[57]

Pressure inside the proton

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Since the proton is composed of quarks confined by gluons, an equivalentpressurethat acts on the quarks can be defined. The size of that pressure and other details about it are controversial.

In 2018 this pressure was reported to be on the order 1035Pa, which is greater than the pressure inside aneutron star.It was said to be maximum at the centre, positive (repulsive) to a radial distance of about 0.6 fm, negative (attractive) at greater distances, and very weak beyond about 2 fm. These numbers were derived by a combination of a theoretical model and experimental Compton scatteringof high-energy electrons.[58][59][60]However, these results have been challenged as also being consistent with zero pressure[61]and as effectively providing the pressure profile shape by selection of the model.[62]

Charge radius in solvated proton, hydronium

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The radius of the hydrated proton appears in theBorn equationfor calculating the hydration enthalpy ofhydronium.

Interaction of free protons with ordinary matter

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Although protons have affinity for oppositely charged electrons, this is a relatively low-energy interaction and so free protons must lose sufficient velocity (andkinetic energy) in order to become closely associated and bound to electrons. High energy protons, in traversing ordinary matter, lose energy by collisions withatomic nuclei,and byionizationof atoms (removing electrons) until they are slowed sufficiently to be captured by theelectron cloudin a normal atom.

However, in such an association with an electron, the character of the bound proton is not changed, and it remains a proton. The attraction of low-energy free protons to any electrons present in normal matter (such as the electrons in normal atoms) causes free protons to stop and to form a new chemical bond with an atom. Such a bond happens at any sufficiently "cold" temperature (that is, comparable to temperatures at the surface of the Sun) and with any type of atom. Thus, in interaction with any type of normal (non-plasma) matter, low-velocity free protons do not remain free but are attracted to electrons in any atom or molecule with which they come into contact, causing the proton and molecule to combine. Such molecules are then said to be "protonated",and chemically they are simply compounds of hydrogen, often positively charged. Often, as a result, they become so-calledBrønsted acids.For example, a proton captured by a water molecule in water becomeshydronium,theaqueouscationH3O+.

Proton in chemistry

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Atomic number

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Inchemistry,the number of protons in thenucleusof an atom is known as theatomic number,which determines thechemical elementto which the atom belongs. For example, the atomic number ofchlorineis 17; this means that each chlorine atom has 17 protons and that all atoms with 17 protons are chlorine atoms. The chemical properties of each atom are determined by the number of (negatively charged)electrons,which for neutral atoms is equal to the number of (positive) protons so that the total charge is zero. For example, a neutral chlorine atom has 17 protons and 17 electrons, whereas a Cl−anionhas 17 protons and 18 electrons for a total charge of −1.

All atoms of a given element are not necessarily identical, however. Thenumber of neutronsmay vary to form differentisotopes,and energy levels may differ, resulting in differentnuclear isomers.For example, there are two stableisotopes of chlorine:35
17Cl
with 35 − 17 = 18 neutrons and37
17Cl
with 37 − 17 = 20 neutrons.

Hydrogen ion

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See also:Hydron (chemistry)
Protium, the most common isotope of hydrogen, consists of one proton and one electron (it has no neutrons). The termhydrogen ion(H+
) implies that that H-atom has lost its one electron, causing only a proton to remain. Thus, in chemistry, the termsprotonandhydrogen ion(for the protium isotope) are used synonymously.

The proton is a unique chemical species, being a bare nucleus. As a consequence it has no independent existence in the condensed state and is invariably found bound by a pair of electrons to another atom.

Ross Stewart,The Proton: Application to Organic Chemistry(1985, p. 1)

In chemistry, the termprotonrefers to the hydrogen ion,H+
.Since the atomic number of hydrogen is 1, a hydrogen ion has no electrons and corresponds to a bare nucleus, consisting of a proton (and 0 neutrons for the most abundant isotopeprotium1
1H
). The proton is a "bare charge" with only about 1/64,000 of the radius of a hydrogen atom, and so is extremely reactive chemically. The free proton, thus, has an extremely short lifetime in chemical systems such as liquids and it reacts immediately with theelectron cloudof any available molecule. In aqueous solution, it forms thehydronium ion,H3O+,which in turn is furthersolvatedby water molecules inclusterssuch as [H5O2]+and [H9O4]+.[63]

The transfer ofH+
in anacid–base reactionis usually referred to as "proton transfer". Theacidis referred to as a proton donor and thebaseas a proton acceptor. Likewise,biochemicalterms such asproton pumpandproton channelrefer to the movement of hydratedH+
ions.

The ion produced by removing the electron from adeuteriumatom is known as adeuteron,not a proton. Likewise, removing an electron from atritiumatom produces atriton.

Proton nuclear magnetic resonance (NMR)

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Also in chemistry, the termproton NMRrefers to the observation of hydrogen-1 nuclei in (mostlyorganic) molecules bynuclear magnetic resonance.This method uses thequantizedspin magnetic momentof the proton, which is due to its angular momentum (orspin), which in turn has a magnitude of one-half the reducedPlanck constant.(). The name refers to examination of protons as they occur inprotium(hydrogen-1 atoms) in compounds, and does not imply that free protons exist in the compound being studied.

Human exposure

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Main article:Effect of spaceflight on the human body
See also:Proton therapy

TheApollo Lunar Surface Experiments Packages(ALSEP) determined that more than 95% of the particles in thesolar windare electrons and protons, in approximately equal numbers.[64][65]

Because the Solar WindSpectrometermade continuous measurements, it was possible to measure how theEarth's magnetic fieldaffects arriving solar wind particles. For about two-thirds of each orbit, theMoonis outside of the Earth's magnetic field. At these times, a typical proton density was 10 to 20 per cubic centimeter, with most protons having velocities between 400 and 650 kilometers per second. For about five days of each month, the Moon is inside the Earth's geomagnetic tail, and typically no solar wind particles were detectable. For the remainder of each lunar orbit, the Moon is in a transitional region known as themagnetosheath,where the Earth's magnetic field affects the solar wind, but does not completely exclude it. In this region, the particle flux is reduced, with typical proton velocities of 250 to 450 kilometers per second. During the lunar night, the spectrometer was shielded from the solar wind by the Moon and no solar wind particles were measured.[64]

Protons also have extrasolar origin from galacticcosmic rays,where they make up about 90% of the total particle flux. These protons often have higher energy than solar wind protons, and their intensity is far more uniform and less variable than protons coming from the Sun, the production of which is heavily affected bysolar proton eventssuch ascoronal mass ejections.

Research has been performed on the dose-rate effects of protons, as typically found inspace travel,on human health.[65][66]To be more specific, there are hopes to identify what specific chromosomes are damaged, and to define the damage, duringcancerdevelopment from proton exposure.[65]Another study looks into determining "the effects of exposure to proton irradiation on neurochemical and behavioral endpoints, includingdopaminergicfunctioning,amphetamine-induced conditioned taste aversion learning, and spatial learning and memory as measured by theMorris water maze.[66]Electrical charging of a spacecraft due to interplanetary proton bombardment has also been proposed for study.[67]There are many more studies that pertain to space travel, includinggalactic cosmic raysand theirpossible health effects,andsolar proton eventexposure.

TheAmerican Biostack and Soviet Biorackspace travel experiments have demonstrated the severity of molecular damage induced by heavy ions onmicroorganismsincludingArtemiacysts.[68]

Antiproton

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Main article:Antiproton

CPT-symmetryputs strong constraints on the relative properties of particles andantiparticlesand, therefore, is open to stringent tests. For example, the charges of a proton and antiproton must sum to exactly zero. This equality has been tested to one part in108.The equality of their masses has also been tested to better than one part in108.By holding antiprotons in aPenning trap,the equality of the charge-to-mass ratio of protons and antiprotons has been tested to one part in6×109.[69]Themagnetic momentof antiprotons has been measured with an error of8×10−3nuclearBohr magnetons,and is found to be equal and opposite to that of a proton.[70]

See also

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References

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