TheInfoList

A proton is a
subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ...
, symbol or , with a positive
electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respectively). Like ch ...
of +1''e''
elementary charge The elementary charge, usually denoted by or sometimes e is the electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric ch ...
and a mass slightly less than that of a
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

. Protons and neutrons, each with masses of approximately one
atomic mass unit The dalton or unified atomic mass unit (symbols: Da or u) is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of ...
, are jointly referred to as "
nucleon In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they under ...
s" (particles present in atomic nuclei). One or more protons are present in the
nucleus ''Nucleus'' (plural nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
of every
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

; they are a necessary part of the nucleus. The number of protons in the nucleus is the defining property of an element, and is referred to as the
atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an essential feature that the photon energy (or frequency) of the electromagnetic radiation emitted (shown) when an electron jumps from one ...
(represented by the symbol ''Z''). Since each
element Element may refer to: Science * Chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all ...
has a unique number of protons, each element has its own unique atomic number. The word ''proton'' is Greek for "first", and this name was given to the hydrogen nucleus by
Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson, (30 August 1871 – 19 October 1937) was a New Zealand-born British physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific resea ...
in 1920. In previous years, Rutherford had discovered that the
hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

nucleus (known to be the lightest nucleus) could be extracted from the nuclei of
nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele and Henry Cavendish had independentl ...

by atomic collisions. Protons were therefore a candidate to be a fundamental or
elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include the fundamental fermions (quarks, leptons, antiquarks, and an ...
, and hence a building block of nitrogen and all other heavier atomic nuclei. Although protons were originally considered elementary particles, in the modern
Standard Model The Standard Model of particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis ...

of
particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that ...
, protons are classified as
hadron In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the ...
s, as are neutrons. Protons are composite particles composed of three
valence quark In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons. The quark model underlies Flavour (particle physics), ...
s: two
up quark The up quark or u quark (symbol: u) is the lightest of all quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, ...

s of charge +''e'' and one
down quark The down quark or d quark (symbol: d) is the second-lightest of all quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (fro ...

of charge −''e''. The
rest mass The invariant mass, rest mass, intrinsic mass, proper mass, or in the case of bound systems simply mass, is the portion of the total mass of an object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** ...
es of quarks contribute only about 1% of a proton's mass. The remainder of a proton's mass is due to quantum chromodynamics binding energy, which includes the
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion (physics), motion. It is defined as the work (physics), work needed to accelerate a body of a given mass from rest to its stated velocity. Having gaine ...
of the quarks and the energy of the
gluon A gluon () is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy ...

fields that bind the quarks together. Because protons are not fundamental particles, they possess a measurable size; the root mean square
charge radius The Root mean square, rms charge radius is a measure of the size of an atomic nucleus, particularly the proton distribution. It can be measured by the scattering of electrons by the nucleus. Relative changes in the mean squared nuclear charge distr ...
of a proton is about 0.84–0.87 fm (or to ). In 2019, two different studies, using different techniques, found the radius of the proton to be 0.833 fm, with an uncertainty of ±0.010 fm. Free protons occur occasionally on Earth: thunderstorms can produce protons with energies of up to several tens of MeV. At sufficiently low temperatures and kinetic energies, free protons will bind to
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...

s. 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 In atomic theory Atomic theory is the scientific theory A scientific theory is an explanation of an aspect of the natural world and universe that has been repeatedly tested and verified in accordance with the scientific method The sc ...
of an atom. The result is a protonated atom, which is a
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemical bonds. A homonuclear molecule, mo ...
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 A hydrogen atom is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touch ...

, which is chemically a
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabol ...
. 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 A molecular cloud, sometimes called a stellar nursery (if star formation Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", Je ...
in
interstellar space Outer space, commonly shortened to space, is the expanse that exists beyond Earth and Earth atmosphere, its atmosphere and between astronomical object, celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a ...
. Free protons are routinely used for accelerators for
proton therapy In the field of medical treatment, proton therapy, or proton radiotherapy, is a type of particle therapy that uses a beam of proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and ...
or various particle physics experiments, with the most powerful example being the
Large Hadron Collider The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider A collider is a type of particle accelerator , a synchrotron collider type particle accelerator at Fermi National Accelerator Laboratory (Fermilab), ...
.

# Description

Protons are spin-
fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. These particles obey the Pauli exclusion principle. Fermions include ...
s and are composed of three valence quarks, making them
baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron list of particles, family of particles; hadrons are composed of ...
s (a sub-type of
hadron In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the ...
s). The two
up quark The up quark or u quark (symbol: u) is the lightest of all quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, ...

s and one
down quark The down quark or d quark (symbol: d) is the second-lightest of all quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (fro ...

of a proton are held together by the
strong force In nuclear physics and particle physics, the strong interaction is the mechanism responsible for the strong nuclear force, and is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction, and ...
, mediated by
gluon A gluon () is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy ...

s. A modern perspective has a proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs of sea quarks. Protons have a positive charge distribution which decays approximately exponentially, with a mean square
radius In classical geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties ...

of about 0.8 fm. Protons and
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

s are both
nucleon In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they under ...
s, which may be bound together by the
nuclear force The nuclear force (or nucleon–nucleon interaction, residual strong force, or, historically, strong nuclear force) is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nuclea ...

to form
atomic nuclei The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger-Marsden experiments, Geiger–Marsden gold foil experiment. After the d ...
. The nucleus of the most common
isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom. The term ...
of the
hydrogen atom A hydrogen atom is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touch ...

(with the
chemical symbol Chemical symbols are the abbreviations used in chemistry for chemical elements, functional groups and chemical compounds. Element symbols for chemical elements normally consist of one or two letters from the Latin alphabet and are written with the f ...
"H") is a lone proton. The nuclei of the heavy hydrogen isotopes
deuterium Deuterium (or hydrogen-2, symbol or , also known as heavy hydrogen) is one of two stable isotopes of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight o ...

and
tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleu ...

contain 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

The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815,
William Prout William Prout Fellow of the Royal Society, FRS (; 15 January 1785 – 9 April 1850) was an English chemist, physician, and natural theologian. He is remembered today mainly for what is called Prout's hypothesis. Biography Prout was born in Ho ...
proposed that all atoms are composed of hydrogen atoms (which he called "protyles"), based on a simplistic interpretation of early values of
atomic weight Relative atomic mass (symbol: ''A'') or atomic weight is a dimensionless physical quantity A physical quantity is a physical property of a material or system that can be Quantification (science), quantified by measurement. A physical quantity ca ...
s (see
Prout's hypothesis Prout's hypothesis was an early 19th-century attempt to explain the existence of the various chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an el ...
), which was disproved when more accurate values were measured. In 1886,
Eugen Goldstein Eugen Goldstein (5 September 1850 – 25 December 1930) was a German physicist. He was an early investigator of discharge tubes, the discoverer of anode rays or canal rays, later identified as positive ions in the gas phase including the hyd ...
discovered
canal rays 200 px, Anode ray tube showing the rays passing through the perforated cathode and causing the pink glow above it. An anode ray (also positive ray or canal ray) is a beam of positive ions that is created by certain types of gas-discharge tubes. ...

(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 of
charge-to-mass ratio The mass-to-charge ratio (''m''/''Q'') is a physical quantity that is most widely used in the electrodynamics of charged particles, e.g. in electron optics and ion optics. It appears in the scientific fields of electron microscopy An electron ...
(e/m), they could not be identified with a single particle, unlike the negative
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...

s discovered by J. J. Thomson.
Wilhelm Wien Wilhelm Carl Werner Otto Fritz Franz Wien (; 13 January 1864 – 30 August 1928) was a German physics, physicist who, in 1893, used theories about heat and electromagnetism to deduce Wien's displacement law, which calculates the Emission (electrom ...

in 1898 identified the hydrogen ion as the particle with the highest charge-to-mass ratio in ionized gases. Following the discovery of the atomic nucleus by
Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson, (30 August 1871 – 19 October 1937) was a New Zealand-born British physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific resea ...
in 1911,
Antonius van den Broek Antonius Johannes van den Broek (4 May 1870, Zoetermeer Zoetermeer () is a city in the Western Netherlands The Netherlands ( nl, Nederland ), informally referred to as Holland, is a country primarily located in Western Europe and partly in the ...
proposed that the place of each element in the
periodic table The periodic table, also known as the periodic table of (the) chemical elements, is a tabular display of the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical element ...

(its atomic number) is equal to its nuclear charge. This was confirmed experimentally by
Henry Moseley Henry Gwyn Jeffreys Moseley (; 23 November 1887 – 10 August 1915) was an English physicist, whose contribution to the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific met ...
in 1913 using X-ray spectra. In 1917 (in experiments reported in 1919 and 1925), Rutherford proved that the hydrogen nucleus is present in other nuclei, a result usually described as the discovery of protons. These experiments began after Rutherford had noticed that, when
alpha particles Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 atomic nucleus, nucleus. They are generally produced in the process of alpha decay, but may ...
were shot into air (mostly nitrogen), his scintillation detectors showed the signatures of typical hydrogen nuclei as a product. 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. After capture of the alpha particle, a hydrogen nucleus is ejected, so that heavy oxygen, not carbon, is the result i.e. Z is not decremented but incremented (see initial proposed reaction below). This was the first reported
nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two atomic nucleus, nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a tr ...
, 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 by
Prout's hypothesis Prout's hypothesis was an early 19th-century attempt to explain the existence of the various chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an el ...
that 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 nucleus H+ 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 The British Science Association (BSA) is a charity and learned society A learned society (; also known as a learned academy, scholarly society, or academic association) is an organization that exists to promote an discipline (academia), ac ...
at its
Cardiff Cardiff (; cy, Caerdydd ) is the capital city, capital and List of urban areas in the United Kingdom, largest city of Wales. It forms a Principal areas of Wales, principal area, officially known as the City and County of Cardiff ( cy, Dinas a S ...

meeting beginning 24 August 1920. Rutherford first proposed (wrongly, see above) that this nitrogen reaction was 14N + α → 14C + α + H+. At the meeting, he was asked by
Oliver Lodge Sir Oliver Joseph Lodge, (12 June 1851 – 22 August 1940) was a British physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, are ...
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). Rutherford later reported that the meeting had accepted his suggestion that the hydrogen nucleus be named the "proton", following Prout's word "protyle". The first use of the word "proton" in the scientific literature appeared in 1920.

# Stability

The free proton (a proton not bound to nucleons or electrons) is a stable particle that has not been observed to break down spontaneously to other particles. 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 in plasmas in which temperatures are too high to allow them to combine with
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...

s. Free protons of high energy and velocity make up 90% of
cosmic ray Cosmic rays are high-energy proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approxi ...
s, which propagate in vacuum for interstellar distances. Free protons are emitted directly from
atomic nuclei The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger-Marsden experiments, Geiger–Marsden gold foil experiment. After the d ...
in some rare types of
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

. Protons also result (along with electrons and
antineutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epist ...
s) from the
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

of free neutrons, which are unstable. The spontaneous decay of free protons has never been observed, and protons are therefore considered stable particles according to the Standard Model. However, some
grand unified theories A Grand Unified Theory (GUT) is a model in particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of natur ...
(GUTs) of particle physics predict that
proton decay#REDIRECT Proton decay In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsi ...

should take place with lifetimes between 1031 to 1036 years and experimental searches have established lower bounds on the
mean lifetime Image:Plot-exponential-decay.svg, upright=1.5, A quantity undergoing exponential decay. Larger decay constants make the quantity vanish much more rapidly. This plot shows decay for decay constant (λ) of 25, 5, 1, 1/5, and 1/25 for x from 0 to 5. A ...
of a proton for various assumed decay products. Experiments at the
Super-Kamiokande Super-Kamiokande (abbreviation of Super-Kamioka Neutrino Detection Experiment, also abbreviated to Super-K or SK; ja, スーパーカミオカンデ) is a Neutrino detector, neutrino observatory located Kamioka Observatory, under Mount Ikeno ne ...
detector in Japan gave lower limits for proton
mean lifetime Image:Plot-exponential-decay.svg, upright=1.5, A quantity undergoing exponential decay. Larger decay constants make the quantity vanish much more rapidly. This plot shows decay for decay constant (λ) of 25, 5, 1, 1/5, and 1/25 for x from 0 to 5. A ...
of for decay to an antimuon and a neutral
pion In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is th ...

, and for decay to a
positron The positron or antielectron is the antiparticle s (left) and antiparticles (right). From top to bottom; electron The electron is a subatomic particle, symbol or , whose electric charge Electric charge is the physical property of mat ...

and a neutral pion. Another experiment at the
Sudbury Neutrino Observatory Artist's concept of SNO's detector. (Courtesy of SNO) The Sudbury Neutrino Observatory (SNO) was a neutrino observatory A neutrino detector is a physics apparatus which is designed to study neutrinos. Because neutrinos only Weak interaction, wea ...
in Canada searched for
gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, it ...
s 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 a proton lifetime of . However, protons are known to transform into
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

s through the process of
electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. Th ...

(also called
inverse beta decay Inverse beta decay, commonly abbreviated to IBD, is a nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two atomic nucleus, nuclei, or a nucleus and an external subatomic particle, collide to pro ...
). For free protons, this process does not occur spontaneously but only when energy is supplied. The equation is: : + → + The process is reversible; neutrons can convert back to protons through
beta decay (the accompanying antineutrino is omitted). The inset shows beta decay of a free neutron. Neither of these depictions shows the intermediate Virtual particle, virtual boson. In nuclear physics, beta decay (''β''-decay) is a type of radioactive ...

, a common form of
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

. In fact, a
free neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

decays this way, with a
mean lifetime Image:Plot-exponential-decay.svg, upright=1.5, A quantity undergoing exponential decay. Larger decay constants make the quantity vanish much more rapidly. This plot shows decay for decay constant (λ) of 25, 5, 1, 1/5, and 1/25 for x from 0 to 5. A ...
of about 15 minutes.

# Quarks and the mass of a proton

In
quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction In nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions. Other forms of n ...
, the modern theory of the nuclear force, most of the mass of protons and
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

s is explained by
special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between Spacetime, space and time. In Albert Einstein's original treatment, the theory is based on two Postulates of ...
. The mass of a proton is about 80–100 times greater than the sum of the rest masses of its three valence
quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge ...

s, while the
gluon A gluon () is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy ...

s have zero rest mass. The extra energy of the
quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge ...

s and
gluon A gluon () is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy ...

s in a proton, as compared to the rest energy of the quarks alone in the
QCD vacuum The QCD vacuum is the vacuum state of quantum chromodynamics (QCD). It is an example of a '' non-perturbative'' vacuum state, characterized by non-vanishing condensates such as the gluon condensate In quantum chromodynamics (QCD), the gluon con ...
, accounts for almost 99% of the proton's mass. The rest mass of a proton is, thus, the
invariant mass Invariant and invariance may refer to: Computer science * Invariant (computer science), an expression whose value doesn't change during program execution ** Loop invariant, invariants used to prove properties of loops * A data type in method ove ...
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 A gluon () is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quark A quark () is a type of elementary particle In particle physics Particle physics (also known as high energy ...

particle field surrounding the quark. These masses typically have very different values. The kinetic energy of the quarks that is a consequence of confinement is a contribution (see ''
Mass in special relativity The word ''mass'' has two meanings in special relativity: ''invariant mass'' (also called rest mass) is an invariant quantity which is the same for all observers in all reference frames; while the ''relativistic mass'' is dependent on the velocity ...
''). Using
lattice QCD Lattice QCD is a well-established non-Perturbation theory (quantum mechanics), perturbative approach to solving the quantum chromodynamics (QCD) theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice (group), latt ...
calculations, the contributions to the mass of the proton are the quark condensate (∼9%, comprised by 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%, comprised by contributions from condensates of all quark flavors). The constituent quark model wavefunction for the proton is :$, p_\uparrow\rangle= \frac \left[2, u_\uparrow d_\downarrow u_\uparrow \rangle + 2, u_\uparrow u_\uparrow d_\downarrow \rangle +2, d_\downarrow u_\uparrow u_\uparrow \rangle - , u_\uparrow u_\downarrow d_\uparrow\rangle -, u_\uparrow d_\uparrow u_\downarrow\rangle -, u_\downarrow d_\uparrow u_\uparrow\rangle -, d_\uparrow u_\downarrow u_\uparrow\rangle -, d_\uparrow u_\uparrow u_\downarrow\rangle-, u_\downarrow u_\uparrow d_\uparrow\rangle\right].$ The internal dynamics of protons 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 a proton directly from the theory to any accuracy, in principle. The most recent calculations claim that the mass is determined to better than 4% accuracy, even to 1% accuracy (see Figure S5 in Dürr ''et al.''). 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 In mathematics, extrapolation is a type of estimation, beyond the original observation range, of the value of a variable on the basis of its relationship with another variable. It is similar to interpolation, which produces estimates between known ...

, which can introduce systematic errors. It is hard to tell whether these errors are controlled properly, because the quantities that are compared to experiment are the masses of the
hadron In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the ...
s, 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 ..." More conceptual approaches to the structure of protons are: the
topological soliton A topological soliton or "toron" occurs when two adjoining structures or space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. Physical space is often conceived in three lin ...
approach originally due to Tony Skyrme and the more accurate AdS/QCD approach that extends it to include a
string theory In physics, string theory is a Mathematical theory, theoretical framework in which the Point particle, point-like particles of particle physics are replaced by Dimension (mathematics and physics), one-dimensional objects called String (physics ...

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 In quantum chromodynamics, the color confinement, confining and strong coupling nature of the theory means that conventional perturbative techniques often fail to apply. The QCD sum rules (or Mikhail Shifman, Shifman–Arkady Vainshtein, Vainshtein ...
, 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.

The problem of defining a radius for an atomic nucleus (proton) is similar to the problem of
atomic radius The atomic radius of a chemical element is a measure of the size of its atoms, usually the mean or typical distance from the center of the Atomic nucleus, nucleus to the boundary of the surrounding Electron shell, shells of electrons. Since the bou ...

, in that neither atoms nor their nuclei have definite boundaries. However, the nucleus can be modeled as a sphere of positive charge for the interpretation of
electron scattering Electron scattering occurs when electrons are deviated from their original trajectory. This is due to the electrostatic forces within matter interaction or, if an external magnetic field is present, the electron may be deflected by the Lorentz for ...

experiments: because there is no definite boundary to the nucleus, the electrons "see" a range of cross-sections, for which a mean can be taken. The qualification of "rms" (for "
root mean square In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
") arises because it is the nuclear cross-section, proportional to the square of the radius, which is determining for electron scattering. The internationally accepted value of a proton's
charge radius The Root mean square, rms charge radius is a measure of the size of an atomic nucleus, particularly the proton distribution. It can be measured by the scattering of electrons by the nucleus. Relative changes in the mean squared nuclear charge distr ...
is (see
orders of magnitude An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually ten, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic ...
for comparison to other sizes). This value is based on measurements involving a proton and an electron (namely,
electron scattering Electron scattering occurs when electrons are deviated from their original trajectory. This is due to the electrostatic forces within matter interaction or, if an external magnetic field is present, the electron may be deflected by the Lorentz for ...

measurements and complex calculation involving scattering cross section based on equation for momentum-transfer cross section), and studies of the atomic energy levels of hydrogen and deuterium. However, in 2010 an international research team published a proton charge radius measurement via the Lamb shift in muonic hydrogen (an exotic atom made of a proton and a negatively charged muon). As a muon is 200 times heavier than an electron, its de Broglie wavelength is correspondingly shorter. This smaller atomic orbital is much more sensitive to the proton's charge radius, so allows more precise measurement. Their measurement of the Root mean square, root-mean-square charge radius of a proton is ", which differs by 5.0 standard deviations from the CODATA value of ". In January 2013, an updated value for the charge radius of a proton——was published. The precision was improved by 1.7 times, increasing the significance of the discrepancy to 7''σ''. The 2014 CODATA adjustment slightly reduced the recommended value for the proton radius (computed using electron measurements only) to , but this leaves the discrepancy at ''σ''. If no errors were found in the measurements or calculations, it would have been necessary to re-examine the world's most precise and best-tested fundamental theory: quantum electrodynamics. The proton radius was a puzzle as of 2017. A resolution came in 2019, when two different studies, using different techniques involving the Lamb shift of the electron in hydrogen, and electron–proton scattering, found the radius of the proton to be 0.833 fm, with an uncertainty of ±0.010 fm, and 0.831 fm. The radius of the proton is linked to the form factor and momentum-transfer cross section. The atomic form factor ''G'' modifies the cross section corresponding to point-like proton. :$\begin R_\text^2 &= -6 _ \\ \frac\ &= _\text G^2\left(q^2\right) \end$ The atomic form factor is related to the wave function density of the target: :$G\left(q^2\right) = \int e^ \psi \left(r\right)^2 \, dr^3$ The form factor can be split in electric and magnetic form factors. These can be further written as linear combinations of Dirac and Pauli form factors. :$\begin G_\text &= F_\text + F_\text \\ G_\text &= F_\text - \tau F_\text \\ \frac &= _ \frac\left\left(G_\text^2\left\left(q^2\right\right) + \fracG_\text^2\left\left(q^2\right\right)\right\right) \end$

## Pressure inside the proton

Since the proton is composed of quarks confined by gluons, an equivalent pressure which acts on the quarks can be defined. This allows calculation of their distribution as a function of distance from the centre using Compton scattering of high-energy electrons (DVCS, for ''deeply virtual Compton scattering''). The pressure is maximum at the centre, about 1035 Pa, which is greater than the pressure inside a neutron star. It is positive (repulsive) to a radial distance of about 0.6 fm, negative (attractive) at greater distances, and very weak beyond about 2 fm.

## Charge radius in solvated proton, hydronium

The radius of the hydrated proton appears in the Born equation for calculating the hydration enthalpy of hydronium.

# Interaction of free protons with ordinary matter

Although protons have affinity for oppositely charged electrons, this is a relatively low-energy interaction and so free protons must lose sufficient velocity (and
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion (physics), motion. It is defined as the work (physics), work needed to accelerate a body of a given mass from rest to its stated velocity. Having gaine ...
) in order to become closely associated and bound to electrons. 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 atomic theory Atomic theory is the scientific theory A scientific theory is an explanation of an aspect of the natural world and universe that has been repeatedly tested and verified in accordance with the scientific method The sc ...
in 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-called Brønsted acids. For example, a proton captured by a water molecule in water becomes hydronium, the aqueous cation .

# Proton in chemistry

## Atomic number

In chemistry, the number of protons in the atomic nucleus, nucleus of an atom is known as the
atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an essential feature that the photon energy (or frequency) of the electromagnetic radiation emitted (shown) when an electron jumps from one ...
, which determines the chemical element to which the atom belongs. For example, the atomic number of chlorine is 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)
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...

s, 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 anion has 17 protons and 18 electrons for a total charge of −1. All atoms of a given element are not necessarily identical, however. The number of neutrons may vary to form different
isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom. The term ...
s, and energy levels may differ, resulting in different nuclear isomers. For example, there are two stable isotopes of chlorine: with 35 − 17 = 18 neutrons and with 37 − 17 = 20 neutrons.

## Hydrogen ion

In chemistry, the term proton refers to the hydrogen ion, . 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 isotope ''protium'' ). 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 the
electron cloud In atomic theory Atomic theory is the scientific theory A scientific theory is an explanation of an aspect of the natural world and universe that has been repeatedly tested and verified in accordance with the scientific method The sc ...
of any available molecule. In aqueous solution, it forms the hydronium ion, H3O+, which in turn is further solvation, solvated by water molecules in Hydrogen ion#Cation (positively charged), clusters such as [H5O2]+ and [H9O4]+. The transfer of in an Brønsted–Lowry acid–base theory, acid–base reaction is usually referred to as "proton transfer". The acid is referred to as a proton donor and the base (chemistry), base as a proton acceptor. Likewise, biochemistry, biochemical terms such as proton pump and proton channel refer to the movement of hydrated ions. The ion produced by removing the electron from a
deuterium Deuterium (or hydrogen-2, symbol or , also known as heavy hydrogen) is one of two stable isotopes of hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight o ...

atom is known as a deuteron, not a proton. Likewise, removing an electron from a
tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleu ...

atom produces a triton.

## Proton nuclear magnetic resonance (NMR)

Also in chemistry, the term "proton NMR" refers to the observation of hydrogen-1 nuclei in (mostly organic chemistry, organic) molecules by nuclear magnetic resonance. This method uses the spin (physics), spin of the proton, which has the value one-half in units of Planck constant, ("h-bar" = ''h''/2π). The name refers to examination of protons as they occur in Hydrogen-1, protium (hydrogen-1 atoms) in compounds, and does not imply that free protons exist in the compound being studied.

# Human exposure

The Apollo Lunar Surface Experiments Packages (ALSEP) determined that more than 95% of the particles in the solar wind are electrons and protons, in approximately equal numbers. Protons also have extrasolar origin from galactic
cosmic ray Cosmic rays are high-energy proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approxi ...
s, 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 by solar proton events such as coronal mass ejections. Research has been performed on the dose-rate effects of protons, as typically found in Human spaceflight, space travel, on human health. 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. 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. Electrical charging of a spacecraft due to interplanetary proton bombardment has also been proposed for study. There are many more studies that pertain to space travel, including galactic cosmic rays and their Health threat from cosmic rays, possible health effects, and solar proton event exposure. The STS-65#Mission parameters, American Biostack and Soviet Biorack space travel experiments have demonstrated the severity of molecular damage induced by heavy ions on microorganisms including Artemia cysts.

# Antiproton

CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, 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 in . The equality of their masses has also been tested to better than one part in . By holding antiprotons in a Penning trap, the equality of the charge-to-mass ratio of protons and antiprotons has been tested to one part in . The magnetic moment of antiprotons has been measured with error of nuclear Bohr magnetons, and is found to be equal and opposite to that of a proton.