# What Is a Neutron? Physics and Chemistry Definition

A neutron is a subatomic particle with a mass of 1 and charge of 0. Neutrons and protons form 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 consists of 1 up quark and 2 down quarks.

The existence of the neutron was proposed by Ernest Rutherford in 1920. Its discovery by James Chadwick in 1932 earned 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 with the name 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 in contrast with the much lighter electron. A neutron is slightly more massive than a proton. Each neutron has a mass of 1.67492729 x 10-27 kg.
• A neutron is a type of fermion because it has a spin of 1/2.
• Neutrons are almost always found within atoms. Although a nucleus sometimes ejects neutrons, 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 a form of ionizing radiation.

### References

• 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