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by Ron Kurtus (updated 8 February 2022)

A neutron is a subatomic particle that is electrically neutral—thus its name. Every element, except Hydrogen, has one or more neutrons in its nucleus.

The neutron seems to be related to holding the nucleus together. A certain number of neutrons makes each nucleus stable, while too many or too few cause instability.

Neutrons have a role in nuclear energy and atomic bombs.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

Characteristics of the neutron

The neutron is an electrically neutral particle that is part of the nucleus of every element except Hydrogen. When a neutron is within the nucleus of an atom, it is relatively stable. But outside the nucleus, neutrons are unstable and decay within about 15 minutes by emitting an electron and antineutrino, turning the neutron into a proton. An antineutrino is the antiparticle of a neutrino, a subatomic particle of extremely small mass.

The atomic weight or atomic mass of a neutron is about 1.0087 u (or amu - atomic mass units). This is slightly more than the atomic mass of a proton, which is 1.0072. Thus, when a free neutron decays into a proton, the extra mass is accounted for by the electron and antineutrino.

The mass of the neutron is 1.675 × 10−27 kg and its quantum spin is ½.

The neutron is made up of one up quark and two down quarks (also called u-quark and d-quark). Since the electrical charge of a d-quark is −1/3 and the charge of a u-quark is +2/3, the resulting charge of a neutron is +2/3 + −1/3 + −1/3 = 0. Since a neutron has three quarks, it is classified as a baryon particle.

Standard view of Neutron consisting of 3 Quarks

Standard view of Neutron consisting of 3 Quarks

Recent studies (2007) have shown the neutron to consist of areas of positive (+) and negative (−) electrical charges. The neutron has an outer edge and an inner core that has a negative charge. Sandwiched between is a region with a positive charge.

New model of Neutron with Quark layers

New model of Neutron with Quark layers

Role of neutrons in the nucleus

The atomic number of an element is determined by the number of protons in its nucleus. The atomic weight or mass of the element is determined by a sum of the atomic mass of its protons and neutrons.


An element may have several different number of neutrons in its nucleus. Although the atomic number is the same, the atomic mass of each would be different, and they are called isotopes of that element.

For example, the most common isotope of Oxygen has 8 protons and 8 neutrons, for an approximate atomic mass of 16. But there also exists isotopes of Oxygen with 9 and even 10 neutrons in the nucleus. These are called O-17 and O-18 for their approximate atomic mass.

(See Chemical Elements and Isotopes in the Chemistry section for more information.)

Purpose of neutron in nucleus

There seems to be a certain number of neutrons in a nucleus required to make the atom stable. Too many neutrons like in O-17, as well as too few neutrons as in O-15, can make the nucleus unstable.

Since protons have a positive (+) electrical charge, they will repel each other, especially in the close proximity of the nucleus. It seems that neutrons have a binding effect on the nucleus and are required to hold it together. It was thought that the reason for neutrons was to keep the protons from flying apart.

But in 1979, evidence of a particle called a gluon was discovered that is supposed to be the "glue" that holds the nucleus together. It is unclear how this particle is supposed to work.

Energy and weapons from neutrons

Neutrons are used in splitting the Uranium atom. Shooting a neutron into Uranium-235 (U-235) makes the nucleus highly unstable, causing it to violently split into two other nuclei, as well as giving off high energy gamma rays and other neutrons. Those neutrons can then cause other U-235 atoms to split, creating a nuclear chain reaction. A controlled reaction is used in nuclear energy. An uncontrolled chain reaction can result in an atomic bomb.

A neutron bomb is a form of atomic bomb that is detonated high above an area. The blast of the explosion does not damage buildings, but the high intensity of the neutrons will kill all living objects in the area. It is a terrible weapon that we hope is never used by anyone.


A neutron is an electrically neutral subatomic particle. Every element except Hydrogen has one or more neutrons in its nucleus. Neutrons have a slightly greater mass than protons. The number of neutrons in a nucleus determines the isotopes of an element. A certain number of neutrons makes each nucleus stable, while too many or too few cause instability. Neutrons have a role in nuclear energy and atomic bombs.

Don't be neutral about things

Resources and references

Ron Kurtus' Credentials


"Charge Densities of the Neutron and Proton" by Gerald A. Miller, University of Washington; Physical Review Letters Vol. 99, 112001 (2007)

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