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The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom, along with neutrons, but is also stable by itself and has a second identity as the hydrogen ion, H+. It is composed of three fundamental particles: two up quarks and one down quark.


Protons are spin-½ fermions and are composed of three quarks, making them baryons. The two up quarks and one down quark of the proton are held together by the strong force, mediated by gluons.

Protons and neutrons are both nucleons, which may be bound by the nuclear force into atomic nuclei. The nucleus of the most common isotope of the hydrogen atom is a single proton (it contains no neutrons). The nuclei of heavy hydrogen (deuterium and tritium) contain neutrons. All other types of atoms are composed of two or more protons and various numbers of neutrons. The number of protons in the nucleus determines the chemical properties of the atom and thus which chemical element is represented; it is the number of both neutrons and protons in a nuclide which determine the particular isotope of an element.


Protons are observed to be stable and their empirically observed half-life is at least 6.6x1035 years The Super-Kamiokande Collaboration, H. Nishino, S. Clark and et al "Search for Proton Decay via p → e+ π0 and p → μ+ π0 in a Large Water Cherenkov Detector", Phys. Rev. Lett. 102, 141801 (2009). Grand unified theories generally predict that proton decay should take place, although experiments so far have only resulted in a lower limit of 1035 years for the proton's lifetime. In other words, proton decay has never been witnessed and the experimental lower bound on the mean proton lifetime ( ) is given by the Sudbury Neutrino Observatorymarker.

However, protons are known to transform into neutrons through the process of electron capture. "When a high energy-proton collides with an atom, it causes the ejection of an electron from the outer layer of the atom.":125 This process does not occur spontaneously but only when energy is supplied. The equation is:

\mathrm{p}^+ + \mathrm{e}^- \rightarrow\mathrm{n} + {\nu}_\text{e} \,
p is a proton,
e is an electron,
n is a neutron, and
νe is an electron neutrino

The 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.

The 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, while a negative Cl ion 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 level may differ forming different isomers. For example, there are two stable isotopes of chlorine: and .

Hydrogen as proton

Since the atomic number of hydrogen is 1, a positive hydrogen ion ( ) has no electrons and corresponds to a bare nucleus with 1 proton (and 0 neutrons for the most abundant isotope ). In chemistry and biology therefore, the word "proton" is commonly used as a synonym for hydrogen ion ( ) or hydrogen nucleus in several contexts:

  1. The transfer of in an acid-base reaction is referred to "proton transfer". The acid is referred to as a proton donor and the base as a proton acceptor.
  2. The hydronium ion (H3O+) in aqueous solution corresponds to a hydrated hydrogen ion. Often the water molecule is ignored and the ion written as simply (aq) or just , and referred to as a "proton". This is the usual meaning in biochemistry, as in the term proton pump which refers to a protein or enzyme which controls the movement of ions across cell membranes.
  3. Proton NMR refers to the observation of hydrogen nuclei in (mostly organic) molecules by nuclear magnetic resonance. This method uses the spin of the proton, which has the value one-half.


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, 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.

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 1919 Rutherford proved that the hydrogen nucleus is present in other nuclei, a result usually described as the discovery of the proton. He noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that this hydrogen could only have come from the nitrogen, and therefore nitrogen must contain hydrogen nuclei. The hydrogen nucleus is therefore present in other nuclei as an elementary particle, which Rutherford named the proton, after the neuter singular of the Greek word for "first", πρῶτον.


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.

Research has been performed on the dose-rate effects of protons, as typically found in space travel, on human health. More specifically, 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 which 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 also demonstrated the severity of damage induced by heavy ions on micro organisms including Artemia cysts.


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 the proton and antiproton must sum to exactly zero. This equality has been tested to one part in 10 . The equality of their masses has also been tested to better than one part in 10 . By holding antiprotons in a Penning trap, the equality of the charge to mass ratio of the proton and the antiproton has been tested to one part in . The magnetic moment of the antiproton has been measured with error of nuclear Bohr magnetons, and is found to be equal and opposite to that of the proton.

See also


  1. W.N.Cottingham and D.A.Greenwood "An Introduction to Nuclear Physics", Cambridge University Press (1986), p.19

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