- For alternative meanings see proton (disambiguation).
In physics, the proton is a subatomic particle with a positive fundamental electric charge of 1.6 × 10-19 coulomb and a mass of 938 MeV (1.6726231 × 10-27 kg, or about 1800 times the mass of an electron). The proton is observed to be stable, with a lower limit on its half-life of about 1035 years, although some theories predict that the proton may decay.
The nucleus of the most common isotope of the hydrogen atom is a single proton. The nuclei of other atoms are composed of protons and neutrons held together by the strong nuclear force. The number of protons in the nucleus determines the chemical properties of the atom and which chemical element it is.
Protons are classified as baryons and are composed of two Up quarks and one Down quark, which are also held together by the strong nuclear force, mediated by gluons. The proton's antimatter equivalent is the antiproton, which has the same magnitude charge as the proton but the opposite sign.
Because the electromagnetic force is many orders of magnitude stronger than the gravitational force, we see that the charge on the proton must be equal to the charge on the electron, otherwise the net repulsion of having an excess of positive or negative charge (depending on which charge was numerically greater - atoms would not be electrically neutral) would cause a noticeable expansion effect on the universe, and indeed any gravitationally aggregated matter (planets, stars, etc.).
In chemistry and biochemistry, the term proton may refer to the hydrogen ion in aqueous solution (in other words, the hydronium ion). In this context, a proton donor is an acid and a proton acceptor a base (see acid-base reaction theories).
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The proton was discovered in 1918 by Ernest Rutherford. When investigating Nitrogen gas, he noticed that when alpha particles were shot into the gas, there were the signs of hydrogen noticed in the scintillation detectors. Rutherford worked out that the only place this hydrogen could have come from was the nitrogen, and therefore nitrogen must contain hydrogen nuclei. He therefore suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle. This he named proton, from protos, the Greek for 'first'.
Protons can exist in spin states. This property is exploited by Nuclear magnetic resonance spectroscopy. In NMR spectroscopy, a magnetic field is applied to a substance in order to detect the shielding around the protons in the nuclei of that substance, which is provided by the surrounding electron clouds. Scientists can use this information to then construct the molecular structure of the molecule under study.