A proton is a subatomic particle, symbol p
or p+, with a positive electric charge of
+1e elementary charge and a mass slightly
less than that of a neutron. Protons and neutrons,
each with masses of approximately one atomic
mass unit, are collectively referred to as
"nucleons".
One or more protons are present in the nucleus
of every 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 (represented
by the symbol Z). Since each element 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 in 1920. In previous
years, Rutherford had discovered that the
hydrogen nucleus (known to be the lightest
nucleus) could be extracted from the nuclei
of nitrogen by atomic collisions. Protons
were therefore a candidate to be a fundamental
particle, and hence a building block of nitrogen
and all other heavier atomic nuclei.
In the modern Standard Model of particle physics,
protons are hadrons, and like neutrons, the
other nucleon (particles present in atomic
nuclei), are composed of three quarks. Although
protons were originally considered fundamental
or elementary particles, they are now known
to be composed of three valence quarks: two
up quarks of charge +2/3e and one down quark
of charge –1/3e. The rest masses of quarks
contribute only about 1% of a proton's mass,
however. The remainder of a proton's mass
is due to quantum chromodynamics binding energy,
which includes the kinetic energy of the quarks
and the energy of the gluon fields that bind
the quarks together. Because protons are not
fundamental particles, they possess a physical
size, though not a definite one; the root
mean square charge radius of a proton is about
0.84–0.87 fm or 0.84×10−15 to 0.87×10−15
m.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.
== Description ==
Protons are spin-½ fermions and are composed
of three valence quarks, making them baryons
(a sub-type of hadrons). The two up quarks
and one down quark of a proton are held together
by the strong force, mediated by gluons.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 of about 0.8 fm.Protons
and neutrons are both nucleons, which may
be bound together by the nuclear force to
form atomic nuclei. The nucleus of the most
common isotope of the hydrogen atom (with
the chemical symbol "H") is a lone proton.
The nuclei of the heavy hydrogen isotopes
deuterium and tritium 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 proposed that all atoms are composed
of hydrogen atoms (which he called "protyles"),
based on a simplistic interpretation of early
values of atomic weights (see Prout's hypothesis),
which was disproved when more accurate values
were measured.
In 1886, Eugen Goldstein discovered canal
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 of charge-to-mass ratio (e/m), they
could not be identified with a single particle,
unlike the negative electrons discovered by
J. J. Thomson. Wilhelm Wien in 1898 identified
the hydrogen ion as particle with highest
charge-to-mass ratio in ionized gases.Following
the discovery of the atomic nucleus by Ernest
Rutherford in 1911, Antonius van den Broek
proposed that the place of each element in
the periodic table (its atomic number) is
equal to its nuclear charge. This was confirmed
experimentally by Henry Moseley in 1913 using
X-ray spectra.
In 1917 (in experiments reported in 1919),
Rutherford proved that the hydrogen nucleus
is present in other nuclei, a result usually
described as the discovery of protons. Rutherford
had earlier learned to produce hydrogen nuclei
as a type of radiation produced as a product
of the impact of alpha particles on nitrogen
gas, and recognize them by their unique penetration
signature in air and their appearance in scintillation
detectors. These experiments were begun when
Rutherford had noticed that, when alpha particles
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
alphas were produced into pure nitrogen gas,
the effect was larger. Rutherford determined
that this hydrogen could have come only from
the nitrogen, and therefore nitrogen must
contain hydrogen nuclei. One hydrogen nucleus
was being knocked off by the impact of the
alpha particle, producing oxygen-17 in the
process. This was the first reported nuclear
reaction, 14N + α → 17O + p. (This reaction
would later be observed happening directly
in a cloud chamber in 1925).
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. 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). 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.Recent research
has shown that thunderstorms can produce protons
with energies of up to several tens of MeV.Protons
are routinely used for accelerators for proton
therapy or various particle physics experiments,
with the most powerful example being the Large
Hadron Collider.
In a July 2017 paper, researchers measured
the mass of a proton to be 1.007276466583+15−29
atomic mass units (the values in parentheses
being the statistical and systematic uncertainties,
respectively), which is lower than measurements
from the CODATA 2014 value by three standard
deviations.
== 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 electrons. Free
protons of high energy and velocity make up
90% of cosmic rays, which propagate in vacuum
for interstellar distances. Free protons are
emitted directly from atomic nuclei in some
rare types of radioactive decay. Protons also
result (along with electrons and antineutrinos)
from the radioactive decay 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 (GUTs) of particle physics predict
that proton decay should take place with lifetimes
between 1031 to 1036 years and experimental
searches have established lower bounds on
the mean lifetime of a 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.
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 a proton lifetime of 2.1×1029 years.However,
protons are known to transform into neutrons
through the process of electron capture (also
called inverse beta decay). For free protons,
this process does not occur spontaneously
but only when energy is supplied. The equation
is:
p+ + e− → n + νeThe process is reversible;
neutrons can convert back to protons through
beta decay, a common form of radioactive decay.
In fact, a free neutron decays this way, with
a mean lifetime of about 15 minutes.
== Quarks and the mass of a proton ==
In quantum chromodynamics, the modern theory
of the nuclear force, most of the mass of
protons and neutrons is explained by special
relativity. The mass of a 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 a 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.
These masses typically have very different
values. As noted, most of a proton's mass
comes from the gluons that bind the current
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 protons (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
9.4 MeV/c2; much of the remainder can be attributed
to the gluons' QCBE. 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, 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 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..."
More conceptual approaches to the structure
of protons 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 ==
The problem of defining a radius for an atomic
nucleus (proton) is similar to the problem
of atomic radius, 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 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") 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 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 (namely, electron
scattering measurements and complex calculation
involving scattering cross section based on
Rosenbluth 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
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". 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, increasing the significance of the
discrepancy to 7σ.
The 2014 CODATA adjustment slightly reduced
the recommended value for the proton radius
(computed using electron measurements only)
to 0.8751(61) fm, but this leaves the discrepancy
at 5.6σ.
The international research team that obtained
this result at the Paul Scherrer Institut
in Villigen includes scientists from the Max
Planck Institute of Quantum Optics, Ludwig-Maximilians-Universität,
the Institut für Strahlwerkzeuge of Universität
Stuttgart, and the University of Coimbra,
Portugal. The team is now attempting to explain
the discrepancy, and re-examining the results
of both previous high-precision measurements
and complex calculations involving scattering
cross section. 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. The proton radius remains
a puzzle as of 2017. Perhaps the discrepancy
is due to new physics, or the explanation
may be an ordinary physics effect that has
been missed.The radius 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.
R
e
2
=
−
6
d
G
e
d
q
2
|
q
2
=
0
d
σ
d
Ω
=
d
σ
d
Ω
|
p
o
i
n
t
G
2
(
q
2
)
{\displaystyle {\begin{aligned}R_{e}^{2}&=-6{{\frac
{dG_{e}}{dq^{2}}}\,{\Bigg \vert }\,}_{q^{2}=0}\\{\frac
{d\sigma }{d\Omega }}\ &={{\frac {d\sigma
}{d\Omega }}\,{\Bigg \vert }\,}_{point}G^{2}(q^{2})\end{aligned}}}
The 
atomic form factor is related to the wave
function density of the target:
G
(
q
2
)
=
∫
e
i
q
r
ψ
(
r
)
2
d
r
3
{\displaystyle G(q^{2})=\int e^{iqr}\psi (r)^{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.
G
m
=
F
D
+
F
P
G
e
=
F
D
−
τ
F
P
d
σ
d
Ω
=
d
σ
d
Ω
|
N
S
1
1
+
τ
(
G
e
2
(
q
2
)
+
τ
ϵ
G
m
2
(
q
2
)
)
{\displaystyle {\begin{aligned}G_{m}&=F_{D}+F_{P}\\G_{e}&=F_{D}-\tau
F_{P}\\{\frac {d\sigma }{d\Omega }}&={{\frac
{d\sigma }{d\Omega }}\,{\Bigg \vert }\,}_{NS}{\frac
{1}{1+\tau }}\left(G_{e}^{2}\left(q^{2}\right)+{\frac
{\tau }{\epsilon }}G_{m}^{2}\left(q^{2}\right)\right)\end{aligned}}}
=== 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 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
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 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 (i.e., 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 are attracted to electrons in
any atom or molecule with which they come
in contact, causing the proton and molecule
to combine. Such molecules are then said to
be "protonated", and chemically they often,
as a result, become so-called Brønsted acids.
== Proton in chemistry ==
=== 
Atomic number ===
In chemistry, the number of protons in the
nucleus of an atom is known as the atomic
number, 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) 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− anion has 17
protons and 18 electrons for a total charge
of −1.
All atoms of a given element are not necessarily
identical, however, as the number of neutrons
may vary to form different isotopes, and energy
levels may differ forming different nuclear
isomers. For example, there are two stable
isotopes of chlorine: 3517Cl with 35 − 17
= 18 neutrons and 3717Cl with 37 − 17 = 20
neutrons.
=== Hydrogen ion ===
In chemistry, the term proton refers 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
isotope protium 11H). 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 of any available molecule.
In aqueous solution, it forms the hydronium
ion, H3O+, which in turn is further solvated
by water molecules in clusters such as [H5O2]+
and [H9O4]+.The transfer of H+ in an acid–base
reaction is usually referred to as "proton
transfer". The acid is referred to as a proton
donor and the base as a proton acceptor. Likewise,
biochemical terms such as proton pump and
proton channel refer to the movement of hydrated
H+ ions.
The ion produced by removing the electron
from a deuterium atom is known as a deuteron,
not a proton. Likewise, removing an electron
from a tritium 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) molecules by nuclear magnetic
resonance. This method uses the spin of the
proton, which has the value one-half. The
name refers to examination of protons as they
occur in 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.
Because the Solar Wind Spectrometer made continuous
measurements, it was possible to measure how
the Earth's magnetic field affects arriving
solar wind particles. For about two-thirds
of each orbit, the Moon is 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
the magnetosheath, 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.
Protons also have extrasolar origin 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,
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
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 possible health effects,
and solar proton event exposure.
The American Biostack and Soviet Biorack space
travel experiments have demonstrated the severity
of molecular damage induced by heavy ions
on micro organisms 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 108. The equality
of their masses has also been tested to better
than one part in 108. 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 6×109. The magnetic
moment of antiprotons has been measured with
error of 8×10−3 nuclear Bohr magnetons,
and is found to be equal and opposite to that
of a proton.
== See also
