A stable, positively charged subatomic particle in the baryon family having a mass 1,836 times that of the electron.
[From Greek prōton, neuter of prōtos, first.]
protonic pro·ton'ic adj.
proton
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pro·ton (prō'tŏn')  |
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A positively charged particle that is the nucleus of the lightest chemical element, hydrogen. The hydrogen atom consists of a proton as the nucleus, to which a single negatively charged electron is bound by an attractive electrical force (since opposite charges attract). The proton is about 1836 times heavier than the electron, so that the proton constitutes almost the entire mass of the hydrogen atom. Most of the interior of the atom is empty space, since the sizes of the proton and the electron are very small compared to the size of the atom. See also Atomic structure and spectra; Electric charge; Hydrogen.
For chemical elements heavier than hydrogen, the nucleus can be thought of as a tightly bound system of Z protons and N neutrons. An electrically neutral atom will then have Z electrons bound comparatively loosely in orbits outside the nucleus. See also Neutron; Nuclear structure.
The numerical values of some overall properties of the proton can be summarized as follows: charge, 1.602 × 10−19 coulomb; mass, 1.673 × 10−27 kg; spin, (½)ℏ (where ℏ is Planck's constant h divided by 2π); magnetic dipole moment, 1.411 × 10−26 joule/tesla; radius, about 10−15 m. See also Fundamental constants; Nuclear moments; Spin (quantum mechanics).
It is instructive to contrast the proton's properties with those of the electron. All of the electron's properties have been found to be those expected of a spin-½ particle which is described by the Dirac equation of quantum mechanics. Such a Dirac particle has no internal size or structure. See also Electron;
By contrast, although it also has a spin of ½, the proton's magnetic moment, which is different from that for a Dirac particle, and its binding with neutrons into nuclei strongly suggest that it has some kind of internal structure, rather than being a point particle. Two different kinds of high-energy physics experiments have been used to study the internal structure of the proton. An example of the first type of experiment is the scattering of high-energy electrons, above say 1 GeV, from a target of protons. The angular pattern and energy distribution of the scattered electrons give direct information about the size and structure of the proton. The second type of high-energy experiment involves the production and study of excited states of the proton, often called baryonic resonances. It has been found that the spectrum of higher-mass states which are produced in high-energy collisions follows a definite pattern. See also Baryon.
In 1963, M. Gell-Mann and, independently, G. Zweig pointed out that this pattern is what would be expected if the proton were composed of three spin-½ particles, quarks, with two of the quarks (labeled u) each having a positive electric charge of magnitude equal to ⅔ of the electron's charge (e), and the other quark (labeled d) having a negative charge of magnitude of ⅓e. Subsequently, the fractionally charged quark concept was developed much further, and has become central to understanding every aspect of the behavior and structure of the proton. See also
An important class of fundamental theories, called grand unification theories (GUTs), makes the prediction that the proton will decay. The predicted lifetime of the proton is very long, about 1030 years or more—which is some 1020 times longer than the age of the universe—but this predicted rate of proton decay may be detectable in practical experiments. See also Grand unification theories.
If the proton is observed to decay, this new interaction will also have profound consequences for understanding of cosmology. The very early times of the big bang (about 10−30 s) are characterized by energies so high that the same grand unified interaction which would allow proton decay would also completely determine the subsequent evolution of the universe. This could then explain the remarkable astrophysical observation that the universe appears to contain only matter and not an equal amount of antimatter. See also
| Dental Dictionary: proton |
(prō′ton)
n
n
An elementary particle having a positive charge equivalent to the negative charge of the electron but possessing a mass approximately 1845 times as great; the proton is a nuclear particle, whereas the electron is extranuclear.
| Measures and Units: proton |
sub-atomic physics Values
[Mohr P. J., Taylor B. N. CODATA Recommended Values of the Fundamental Physical Constants: 2002 (to be published)]
[Mohr P. J., Taylor B. N. Rev. Mod. Phys. Vol. 72:351-495 (2000)]
[Mohr P. Phys. Today Vol. 53:7, 11-16 (2000)]
[For latest recommended values, see
| proton charge = elementary charge, | ||
| proton gyromagnetic ratio (γp) | 2.675 222 05(23) × 108 s-1·T-1 | 8.6 × 10-8, |
| proton magnetic moment (μP) | 1.410 606 71(12) × 10-26 J·T-1 | 8.7 × 10-8, |
| proton mass (mp) | 1.672 621 71(29) × 10-27 kg | 1.7 × 10-7, |
| = 1 836.152 672 61(85) me | 4.6 × 10-10. |
| Britannica Concise Encyclopedia: proton |
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| Columbia Encyclopedia: proton |
proton, elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom. The positive charge of the nucleus of any atom is due to its protons. Every atomic nucleus contains one or more protons; the number of protons, called the atomic number, is different for every element (see periodic table). The mass of the proton is about 1,840 times the mass of the electron and slightly less than the mass of the neutron. The total number of nucleons, as protons and neutrons are collectively called, in any nucleus is the mass number of the nucleus. The existence of the nucleus was postulated by Ernest Rutherford in 1911 to explain his experiments on the scattering of alpha particles; in 1919 he discovered the proton as a product of the disintegration of the atomic nucleus. The proton and the neutron are regarded as two aspects or states of a single entity, the nucleon. The proton is the lightest of the baryon class of elementary particles. The proton and other baryons are composed of triplets of the elementary particle called the quark. A proton, for instance, consists of two quarks called up and one quark called down, a neutron consists of two down quarks and an up quark. The antiparticle of the proton, the antiproton, was discovered in 1955; it has the same mass as the proton but a unit negative charge and opposite magnetic moment. Protons are frequently used in a particle accelerator as either the bombarding (accelerated) particle, the target nucleus, or both. The possibility that the proton may have a finite lifetime has recently come under examination. If the proton does indeed decay into lighter products, however, it takes an extremely long time to do so; experimental evidence suggests that the proton has a lifetime of at least 1031 years.
| Science Dictionary: proton |
| Veterinary Dictionary: proton |
An elementary particle of mass number 1, with a positive charge equal to the negative charge of the electron; a constituent particle of every nucleus, the number of protons in the nucleus of each atom of a chemical element being indicated by its atomic number.
- p. pump — see sodium pump.
- p. pump inhibitor — a class of therapeutic agents which are used to counteract the effects of proton pumps in tissues and organs, particularly used in counteracting gastric hypersecretion and acidity.
| Wikipedia: Proton |
For other uses, see Proton (disambiguation).
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| Classification: | Baryon |
| Composition: | 2 up, 1 down |
| Family: | Fermion |
| Group: | Hadron |
| Interaction: | Gravity, Electromagnetic, Weak, Strong |
| Antiparticle: | Antiproton |
| Theorized: | William Prout (1815) |
| Discovered: | Ernest Rutherford (1919) |
| Symbol(s): | p, p+, N+ |
| Mass: | 1.672621637(83)×10−27 kg
938.272013(23) MeV/c2 1.00727646677(10) u [1] |
| Mean lifetime: | >2.1×1029 years (stable) |
| Electric charge: | +1 e. 1.602176487(40) × 10-19 C[1] |
| Charge radius: | 0.875(7) fm |
| Electric dipole moment: | <5.4×10−24 e cm |
| Electric polarizability: | 1.20(6)×10−3 fm3 |
| Magnetic moment: | 2.792847351(28) μN |
| Magnetic polarizability: | 1.9(5)×10−4 fm3 |
| Spin: | 1⁄2 |
| Isospin: | 1⁄2 |
| Parity: | +1 |
| Condensed: | I(JP) = 1⁄2(1⁄2+) |
The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, 1H+. It is composed of 3 even more fundamental particles comprising two up quarks and one down quark.[2]
Contents |
Description
Protons are spin-1/2 fermions and are composed of three quarks[3], making them baryons. The two up quarks and one down quark of the proton are held together by the strong force, mediated by gluons.[2]
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 have a positive charge.
Stability
Protons are observed to be stable and their theoretical minimum half-life is 1×1036 years. 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 (2.1×1029 years) is put by the Sudbury Neutrino Observatory[4].
However, protons are known to transform into neutrons through the process of electron capture. This process does not occur spontaneously but only when energy is supplied. The equation is:
where
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. Again for chlorine as an example, there are two stable isotopes - 35Cl with 35 nucleons which are 17 protons and 35-17 = 18 neutrons, and 37Cl with 17 protons and 37-17 = 20 neutrons. Other isotopes of chlorine are radioactive.
Hydrogen as proton
Since the atomic number of hydrogen is 1, a positive hydrogen ion (H+) has no electrons and corresponds to a bare nucleus with 1 proton (and 0 neutrons for the most abundant isotope 1H). In chemistry and biology therefore, the word "proton" is commonly used as a synonym for hydrogen ion (H+) or hydrogen nucleus in several contexts:
- The transfer of H+ 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.
- 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 H+(aq) or just H+, 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 H+ ions across cell membranes.
- 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.
History
Ernest Rutherford is generally credited with the discovery of the proton. In 1918 Rutherford noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen, and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle.
See also: William Prout and Prout's hypothesis
Prior to Rutherford, Eugen Goldstein had observed canal rays, which were composed of positively charged ions. After the discovery of the electron by J.J. Thomson, Goldstein suggested that since the atom is electrically neutral there must be a positively charged particle in the atom and tried to discover it. He used the "canal rays" observed to be moving against the electron flow in cathode ray tubes. After the electron had been removed from particles inside the cathode ray tube they became positively charged and moved towards the cathode. Most of the charged particles passed through the cathode, it being perforated, and produced a glow on the glass. At this point, Goldstein believed that he had discovered the proton.[5] When he calculated the ratio of charge to mass of this new particle (which in case of the electron was found to be the same for every gas that was used in the cathode ray tube) was found to be different when the gases used were changed. The reason was simple. What Goldstein assumed to be a proton was actually an ion. He gave up his work there, but promised that "he would return." However, he was widely ignored.
The proton is named after the neuter singular of the Greek word for "first", πρῶτον.
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.[6][7]
- "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."[6]
Research has been performed on the dose-rate effects of protons, as typically found in space travel, on human health.[7][8] More specifically, there are hopes to identify what specific chromosomes are damaged, and to define the damage, during cancer development from proton exposure.[7] 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."[8] Electrical charging of spacecraft by exposure to interplanetary protons has also been studied.[9] There are many more studies which pertain to space travel, including galactic cosmic rays and their possible health effects, and solar proton event exposure.
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 the 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 the proton and the antiproton has been tested to one part in 9×1011. The magnetic moment of the antiproton has been measured with error of 8×10−3 nuclear Bohr magnetons, and is found to be equal and opposite to that of the proton.
See also
References
- ^ a b C. Amsler et al., "Review of Particle Physics" Physics Letters B667, 1 (2008)
- ^ a b W.N.Cottingham and D.A.Greenwood "An Introduction to Nuclear Physics", Cambridge University Press (1986), p.19
- ^ Adair, Robert K.: "The Great Design: Particles, Fields, and Creation.", page 214. New York: Oxford University Press, 1989.
- ^ SNO Collaboration, S.N. Ahmed et al., "Constraints on nucleon decay via invisible modes from the Sudbury Neutrino Observatory", Phys. Rev. Lett. 92 (2004) 102004
- ^ Gilreath, Esmarch S.: "Fundamental Concepts of Inorganic Chemistry.", page 5. New York: McGraw–Hill, 1958.
- ^ a b "Apollo 11 Mission" Lunar and Planetary Institute, 2009. Accessed 12 June 2009.
- ^ a b c "Space Travel and Cancer Linked? Stony Brook Researcher Secures NASA Grant to Study Effects of Space Radiation" December 12, 2007. Brookhaven National Laboratory News. Stony Brook, N.Y., Accessed 12 June 2009
- ^ a b Shukitt-Hale, B., Szprengiel, A., Pluhar, J., Rabin, B. M. & Joseph, J. A. "The effects of proton exposure on neurochemistry and behavior". Elsevier Ltd on behalf of COSPAR. Article summary published by bilioteca.net. Accessed 12 June 2009.
- ^ N. W. Green and A. R. Frederickson. "A Study of Spacecraft Charging due to Exposure to Interplanetary Protons". Jet Propulsion Laboratory, California Institute of Technology. Accessed 12 June 2009.
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| Translations: Proton |
Ελληνική (Greek)
n. - (φυσ.) πρωτόνιο
Português (Portuguese)
n. - próton (m)
Svenska (Swedish)
n. - proton (fys.)
中文（简体）(Chinese (Simplified))
质子
中文（繁體）(Chinese (Traditional))
n. - 質子
العربيه (Arabic)
(الاسم) جزيئه ذريه مشحونه موجبا
עברית (Hebrew)
n. - פרוטון - חלקיק בעל מטען חשמלי חיובי בגרעין האטום
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Mentioned in
- nucleon
- proton-proton scattering (nuclear physics)
- aprotic solvent (chemistry)
- Conjugate Base (science)
- proton drip-line (nuclear physics)
- p (abbreviation)
- photoproton (nuclear physics)
- protonate (chemistry)
- proton capture (nuclear physics)
- protonium (atomic physics)
- Conjugate Acid (science)
- acid-base pair (chemistry)
- intermediate-energy proton-proton scattering (nuclear physics)
- polyprotic
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