Proton

For other uses, see Proton (disambiguation).

Proton

The quark structure of the proton.
Classification	fail
Composition	2 up, 1 down
Family	QuarkFermion
Interactions	Gravity, Electromagnetic, Weak, Strong
Symbol	p , p+ , N+
Antiparticle	Antiproton
Theorized	William Prout (1815)
Discovered	Ernest Rutherford (1919)
Mass	1.67262171(29)×10−27 kg 938.272029(80) MeV/c2 1.00727646688(13) u
Mean lifetime	>1.9×1029 years (stable)
Electric charge	1.60217653(14)×10−19 C
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	½
Isospin	½
Parity	+1
Condensed	I(JP) = ½(½+)

The proton (Greek πρῶτον / proton "first") is a subatomic particle with an electric charge of one positive fundamental unit (1.60217653(14)×10⁻¹⁹ C), a diameter of about 1.65×10⁻¹⁵ m, and a mass of 938.27231(28) MeV/c² (1.00727646688(13) u, 1.6726×10⁻²⁷ kg), or about 1836 times the mass of an electron.

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.

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:

$\mathrm {p} ^{+}+\mathrm {e} ^{-}\rightarrow \mathrm {n} +{\nu }_{e}\,$

where

p is a proton,

e is an electron,

n is a neutron, and

$\nu _{e}$ 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.

In chemistry and biochemistry

In chemistry and biochemistry, the word "proton" is commonly used as a synonym for hydrogen ion (H⁺) or hydrogen nucleus in several contexts:

1. 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.
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 H⁺(aq) or just H⁺, and referred to as a "proton".
3. Proton NMR refers to the observation of hydrogen nuclei in (mostly organic) molecules by nuclear magnetic resonance.

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, Eugene 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. 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.

Description

Protons are spin −1/2 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.

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.

High-energy physics

Due to their stability and large mass (relative to electrons), protons are well suited to use in particle colliders such as the Large Hadron Collider at CERN and the Tevatron at Fermilab. Protons also make up a large majority of the cosmic rays which impinge on the Earth's atmosphere. Such high-energy proton collisions are more complicated to study than electron collisions, due to the composite nature of the proton. Understanding the details of proton structure requires quantum chromodynamics.

See also

• particle physics
• subatomic particle
• quark model
• neutron
• electron
• proton-proton chain reaction
• proton pump
• proton pump inhibitor
• proton therapy
• list of particles
• fermion field
• proton decay

External links

• Particle Data Group
• Large Hadron Collider