nucleon

nucleon, term applying to both the proton and the neutron, the two constituents of atomic nuclei. The nucleon may be considered a single particle, of which the proton and the neutron are two different states. See atom; elementary particles.

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nucleon

a proton or neutron, esp one present in an atomic nucleus

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Nucleon

The collective name for a proton or a neutron. These subatomic particles are the principal constituents of atomic nuclei and therefore of most matter in the universe. The proton and neutron share many characteristics. They have the same intrinsic spin, nearly the same mass, and similar interactions with other subatomic particles, and they can transform into one another by means of the weak interactions. Hence it is often useful to view them as two different states or configurations of the same particle, the nucleon. Nucleons are small compared to atomic dimensions and relatively heavy. Their characteristic size is of order 1/10,000 the size of a typical atom, and their mass is of order 2000 times the mass of the electron.

The proton and neutron differ chiefly in their electromagnetic properties. The proton has electric charge +1, the opposite of the electron, while the neutron is electrically neutral. They have significantly different intrinsic magnetic moments. Because the neutron is slightly heavier than the proton, roughly 1 part in 1000, the neutron is unstable, decaying into a proton, an electron, and an antineutrino with a characteristic lifetime of approximately 900 s. Although some unified field theories predict that the proton is unstable, no experiment has detected proton decay.

The complex forces between nucleons and the discovery during the 1950s of many similar subatomic particles led physicists to suggest that nucleons might not be fundamental particles. During the late 1960s and 1970s, inelastic electron and neutrino scattering experiments indicated that nucleons are composed of pointlike particles with spin ½ and electric charges that are fractions of the charge on the electron. Particles with similar properties, named quarks, had been hypothesized in the early 1960s to explain other regularities among the properties of hadrons. In the early 1970s, it became clear that nucleons and other hadrons are indeed bound states of quarks. See Hadron, Nuclear structure

Quarks are believed to be fundamental particles without internal structure. The proton consists of two up-type quarks and one down-type quark (uud), while the neutron consists of ddu. Quarks are bound into nucleons by strong forces carried by gluons. The nucleon contains ambient gluon fields in somewhat the same way that the atom contains ambient electromagnetic fields. Because quarks and gluons are much less massive than the nucleon itself, their motion inside the nucleon is relativistic, making quark-antiquark pair creation a significant factor. Thus the nucleon contains fluctuating quark-antiquark pairs in addition to quarks and gluons. The theory of quark-gluon interactions is known as quantum chromodynamics (QCD), in analogy to the quantum theory of electrodynamics (QED). See Elementary particle, Gluons, Neutron, Proton, Quantum chromodynamics, Quantum electrodynamics, Quarks