What Is a Neutron? Physics and Chemistry Definition

The neutron is a neutral subatomic particle found in the atomic nucleus.
The neutron is a neutral subatomic particle found in the atomic nucleus.

The neutron is a subatomic particle with a mass of 1 and charge of 0. Neutrons are found with protons in the atomic nucleus. While the number of protons in a atom determines its element, the number of neutrons determines its isotope.

Although a neutron has a net neutral electrical charge, it consists of charged components which cancel each other out with respect to charge. Each neutron is a type of subatomic particle called a baryon that is composed of 1 up quark and 2 down quarks.

The existence of the neutron was proposed by Ernest Rutherford in 1920. It was discovered by James Chadwick in 1932, earning him the Nobel Prize in Physics in 1935. Chadwick and his doctoral student Maurice Goldhaber accurately measured the mass of the neutron in 1935.

Neutron Facts

  • Nuclear fission and nuclear fusion are two nuclear reactions that release large quantities of neutrons.
  • Atoms of every element contain neutrons, except the most common isotope of hydrogen. The hydrogen isotope called protium (“normal” hydrogen) consists of a proton and an electron, but no neutron. A deuterium atom contains one proton, while a tritium atom contains two neutrons.
  • The mass of a proton and a neutron are comparable, especially compared with the much lighter electron, but a neutron is slightly more massive than a proton. A neutron has a mass of 1.67492729 x 10-27 kg.
  • A neutron is considered a type of fermion because it has a spin of 1/2.
  • Neutrons are almost always found within atoms. Although it’s possible to eject neutrons from the nucleus, the free particles don’t last long before reacting with other atoms. A free neutron has a lifetime of about 15 minutes.
  • Free neutrons are considered a form of ionizing radiation.


  • Byrne, James (2011). Neutrons, Nuclei and Matter: An Exploration of the Physics of Slow Neutrons. Mineola, New York: Dover Publications. ISBN 0486482383.
  • Chadwick, J.; Goldhaber, M. (1934). “A nuclear photo-effect: disintegration of the diplon by gamma rays”. Nature. 134 (3381): 237–238. doi:10.1038/134237a0