The quark structure of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.
The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass slightly larger than that of a proton. Protons and neutrons, each with mass approximately one atomic mass unit, constitute the nucleus of an atom, and they are collectively referred to as "nucleons". Their properties and interactions are described by nuclear physics.
The nucleus consists of a number of protons, or atomic number with symbol Z, and a number of neutrons, or neutron number with symbol N. The atomic number defines the chemical properties of the atom, and the neutron number determines the isotope or nuclide. Thee terms isotope or nuclide are often used synonymously, but they refer to chemical or nuclear properties, respectively. The atomic mass number, symbol A, equals Z+N. For example, carbon has atomic number 6, and its abundant carbon-12 isotope has 6 neutrons, whereas its rare carbon-13 isotope has 7 neutrons. Some elements occur in nature with only one stable isotope, such as fluorine. Other elements occur as many stable isotopes, such as tin with ten stable isotopes. Even though it is not a chemical element, the neutron is included in the table of nuclides.
Within the nucleus, protons and neutrons are bound together through the nuclear force, and neutrons are required for the stability of nuclei. Neutrons are produced copiously in nuclear fission and fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes.
The neutron is essential to the production of nuclear power. After the neutron was discovered in 1932 CE, as quickly realized that neutrons might act to form a nuclear chain reaction. In the 1930s CE, neutrons were used to produce many different types of nuclear transmutations. When nuclear fission was discovered in 1938 CE, it became clear that, if a fission event produced neutrons, each of these neutrons might cause further fission events, etc., in a cascade known as a chain reaction. These events and findings led to the first self-sustaining nuclear reactor (Chicago Pile-1, 1942 CE) and the first nuclear weapon (Trinity, 1945 CE).
Free neutrons, or individual neutrons free of the nucleus, are effectively a form of ionizing radiation, and as such, are a biological hazard, depending upon dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray muons, and by the natural radioactivity of spontaneously fissionable elements in the Earth's crust. Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments.