The neutron is an electrically neutral subatomic particle that, together with protons, forms atomic nuclei. It has a rest mass of 1.00866491595 u (≈ 939.565 MeV/c²) and a magnetic moment of –1.913 μN. Discovered by James Chadwick in 1932, free neutrons undergo beta decay with a mean lifetime of about 878 seconds.
Last updated February 10, 2026
Neutron (neutral subatomic particle)\n\n## Overview\nThe neutron is an electrically neutral baryon with spin ½ that, together with the proton, composes the atomic nucleus. Its rest mass is 1.00866491595 u (≈ 939.565 MeV/c²) and its magnetic moment is –1.913 μN Britannica.\n\n## Physical Properties\n- Mass: 1.00866491595 u (PDG 2024)\n- Mass (MeV): 939.56542052 MeV/c² (PDG)\n- Magnetic Moment: –1.91304273 μN (PDG)\n- Mean Lifetime: 878.4 s for a free neutron (PDG)\n- Charge: Zero within 10⁻²² e (PDG)\n- Quark Content: two down quarks and one up quark (udd) Britannica.\n\n## Decay\nA free neutron undergoes β⁻ decay: \n\n`n → p + e⁻ + \bar{ν}_e`\n\nThe process has a mean lifetime of ~878 seconds, releasing a proton, an electron and an antineutrino. This decay underlies many nuclear processes and radiometric dating techniques.\n\n## Role in Nuclei\nNeutrons provide the strong nuclear force that binds protons together, offsetting electrostatic repulsion. Varying the neutron‑to‑proton ratio generates isotopes, as explained in Britannica – Isotope. Stable isotopes exhibit no spontaneous decay, whereas neutron‑rich nuclei may β‑decay toward stability.\n\n## Applications\n- Nuclear Reactors: Moderators such as water, heavy water or graphite slow fast neutrons to thermal energies, increasing fission probability NRC.\n- Neutron Activation Analysis: Samples irradiated with neutrons become radioactive, allowing quantitative elemental analysis IAEA.\n- Neutron Scattering: Provides insights into crystal structures, magnetism and molecular dynamics; facilities like ISIS exploit this technique ISIS.\n\n## Astrophysical Significance\nIn the cores of massive stars, electron capture produces neutron‑rich matter that collapses into a Neutron Star. These objects, about 20 km in radius, contain matter at nuclear density and exhibit magnetic fields up to 10¹² gauss.\n\n## Discovery\nJames Chadwick experimentally identified the neutron in 1932, a breakthrough recognized with the 1935 Nobel Prize in Physics Nobel Prize. The discovery clarified the composition of atomic nuclei and opened the field of nuclear physics.\n\n## Related Concepts\nProton, Atomic Nucleus, Beta Decay, James Chadwick, Neutron Star, Neutron Scattering
The neutron is an electrically neutral subatomic particle that, together with protons, forms atomic nuclei. It has a rest mass of 1.00866491595 u (≈ 939.565 MeV/c²) and a magnetic moment of –1.913 μN. Discovered by James Chadwick in 1932, free neutrons undergo beta decay with a mean lifetime of about 878 seconds.
Last updated February 10, 2026
Neutron (neutral subatomic particle)\n\n## Overview\nThe neutron is an electrically neutral baryon with spin ½ that, together with the proton, composes the atomic nucleus. Its rest mass is 1.00866491595 u (≈ 939.565 MeV/c²) and its magnetic moment is –1.913 μN Britannica.\n\n## Physical Properties\n- Mass: 1.00866491595 u (PDG 2024)\n- Mass (MeV): 939.56542052 MeV/c² (PDG)\n- Magnetic Moment: –1.91304273 μN (PDG)\n- Mean Lifetime: 878.4 s for a free neutron (PDG)\n- Charge: Zero within 10⁻²² e (PDG)\n- Quark Content: two down quarks and one up quark (udd) Britannica.\n\n## Decay\nA free neutron undergoes β⁻ decay: \n\n`n → p + e⁻ + \bar{ν}_e`\n\nThe process has a mean lifetime of ~878 seconds, releasing a proton, an electron and an antineutrino. This decay underlies many nuclear processes and radiometric dating techniques.\n\n## Role in Nuclei\nNeutrons provide the strong nuclear force that binds protons together, offsetting electrostatic repulsion. Varying the neutron‑to‑proton ratio generates isotopes, as explained in Britannica – Isotope. Stable isotopes exhibit no spontaneous decay, whereas neutron‑rich nuclei may β‑decay toward stability.\n\n## Applications\n- Nuclear Reactors: Moderators such as water, heavy water or graphite slow fast neutrons to thermal energies, increasing fission probability NRC.\n- Neutron Activation Analysis: Samples irradiated with neutrons become radioactive, allowing quantitative elemental analysis IAEA.\n- Neutron Scattering: Provides insights into crystal structures, magnetism and molecular dynamics; facilities like ISIS exploit this technique ISIS.\n\n## Astrophysical Significance\nIn the cores of massive stars, electron capture produces neutron‑rich matter that collapses into a Neutron Star. These objects, about 20 km in radius, contain matter at nuclear density and exhibit magnetic fields up to 10¹² gauss.\n\n## Discovery\nJames Chadwick experimentally identified the neutron in 1932, a breakthrough recognized with the 1935 Nobel Prize in Physics Nobel Prize. The discovery clarified the composition of atomic nuclei and opened the field of nuclear physics.\n\n## Related Concepts\nProton, Atomic Nucleus, Beta Decay, James Chadwick, Neutron Star, Neutron Scattering
