Proton

The proton is a subatomic particle with the symbol p or p+ and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number.

In the standard model of particle physics, the proton is a hadron, composed of quarks. Prior to that model becoming a consensus in the physics community, the proton was considered a fundamental particle. A proton is composed of two up quarks and one down quark, with the rest mass of the quarks thought to contribute only about 1% of the protons weight.

Because the proton is not a fundamental particle, it possesses a physical size—although this is not perfectly well-defined since the surface of a proton is defined by forces that do not come to an abrupt end, and is therefore somewhat fuzzy. The proton is about 1.6–1.7 fm in diameter.ton's mass. The remainder of the proton mass is due to the kinetic energy of the quarks and to the energy of the gluon fields that bind them together.

The free proton (a proton not bound to nucleons or electrons) is stable and is found naturally in a number of situations. Free protons exist in plasmas in which temperatures are too high to allow them to combine with electrons. Free protons of high energy and velocity make up 90% of cosmic rays, which propagate in vacuum for interstellar distances. Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay, and also result from the decay of free neutrons, which are unstable