Illustration of an atomic nucleus as a series of red and white circles, orbited by electrons represented by white circles.
Computer illustration of an atomic model. Science Photo Library/Getty Images

Definition: A neutron is a subatomic particle contained in the atomic nucleus. It has no net electric charge, unlike the proton's positive electric charge. The number of neutrons in an atomic nucleus determines the isotope of that element.

Discovery of the Neutron

The first hint of the neutron's existence came in 1930, when Walther Bothe and H. Becker found that when alpha radiation fell on elements like lithium and boron a new form of radiation was emitted.

Initially, this radiation was believed to be a type of gamma radiation, but it was more penetrating than any known gamma radiation. Work by Irene Joliot-Curie and Frederic Joliot in 1932, though not disproving the gamma radiation hypothesis, did not particularly support it either.

In 1932, James Chadwick proved that these results couldn't be explained by gamma rays and proposed an alternate explanation of uncharged particles approximately the same size as a proton. He was able to experimentally verify this conjecture and thus prove that the neutron existed.

Neutron Details

The neutron is a hadron composed of three quarks, one up quark and two down quarks.

The mass of the neutron is slightly larger than that of the proton.

The neutron does not exist long outside of the atomic nucleus, only a mere 885 seconds (about 15 minutes) on average.

  • Mass: 1.67492729 × 10−27 kg
  • Charge: 0
  • Spin: 1/2

Because the neutron has a 1/2 spin, it is a fermion.