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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/ c² ( 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 decayFor a long time, protons were thought to be stable—that the laws of physics would not allow for a proton (which is baryonic matter) to spontaneously decay into a positron and photons (non-baryonic matter) because of conservation of the baryon number..

The nucleusThe nucleus atomic nucleus is the center of an atom. It is composed of one or more protons and usually some neutrons as well. The number of protons in an atom's nucleus is called the atomic number, and determines which element the atom is (for example hyd of the most common isotope of the hydrogenhydrogen helium H Li Full table General Name, Symbol, NumberHydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1 (IA), 1 , s Density, Hardness 0. 0899 kg/m3, NA Appearance colorless Atomic properties Atomic weight 1. 00794 amu Atomic radius (ca atomFor alternative meanings see atom (disambiguation). An atom is a microscopic structure found in all ordinary matter around us. Atoms are composed of 3 types of subatomic particles: electrons, which have a negative charge; protons, which have a positive ch is a single proton. The nuclei of other atoms are composed of protons and neutronNeutron Classification Subatomic particle Fermion Hadron Baryon Nucleon Neutron Properties Mass: 940 MeV/ c 2 Electric Charge: 0 C Spin: 1/2 In physics, the neutron is a subatomic particle with no net electric charge and a mass of 940 MeV/ c 2 ( kg; verys 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 this context, a proton donor is an acid and a proton acceptor a base (see acid-base reaction theories).