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Atoms and Molecules




Bonds and Octet Rules
Chemical Equations
The Mole




Functional Groups
Periodic Table


Practice Quiz I
Practice Quiz II
Practice Quiz III


The neutron is one of the three principal sub-atomic particles in the atom. Unlike the proton and the electron, the neutron carries no charge. Neutrons have just slightly less mass than protons. To make things easier, however, scientists attribute to the neutron a mass equal to that of a proton, or 1 amu.

A neutron in an atom

This is an imaginary construct being used for illustrative purposes.

Neutrons are found only in the nucleus of the atom, where they play an important role in keeping the atom stable. Since protons are positively charged, there is an incredibly strong repulsive force between all the protons in a nucleus. To counteract this strong repulsive force, neutrons apply an even stronger force on the protons in a nucleus. This force can only be applied over very small distances, so neutrons have to pack very tightly against the protons in a nucleus, which is why a nucleus is so dense.

Neutrons keep the protons in a nucleus stable

This is an imaginary construct being used for illustrative purposes

The number of neutrons in an atom of an element can vary.  Atoms of an element that contain varying numbers of neutrons are called isotopes of that element.  Every isotope of an element contains the same number of protons, but different numbers of neutrons.  Three isotopes of hydrogen are shown below.

Sequence of three hydrogen isotopes

For every isotope of an element the number of protons in the nucleus is always constant.  For example, the number of protons in any atom of hydrogen is 1, and the number of protons in any atom of helium is 2.

The number of protons in the nucleus of an atom is called its atomic number (Z), which identifies the atom.  The sum of the number of protons and neutrons in the nucleus of an atom is called the mass number (A), or nucleon number.  For isotopes of an element, the mass number is always different (because the number of neutrons changes and neutrons contribute to the mass number).

When an atom is denoted by its symbol followed by a dash followed by a number (for example, C-12), that number is the mass number.  The complete nuclear symbol for the isotope C-12 (carbon-12)--which contains 6 protons and 6 neutrons in its nucleus--is , where 6 denotes the atomic number (Z) of carbon and 12 denotes its mass number (A).

For most naturally occuring elements, one isotope will be far more common than any other. For example, carbon-12 comprises 98.93% of naturally occurring carbon.

The discovery of the neutron was made in 1932 by English physicist James Chadwick. Chadwick had heard of an experiment that involved firing alpha particles at beryllium atoms to produce high levels of radiation; this radiation proved to be strong enough to push protons out of the nuclei of various atoms. This proton ejection contradicted the then current scientific beliefs about energy. Chadwick, unwilling to believe that the laws of physics were wrong, investigated the nature of this radiation. He discovered that the “radiation” was not radiation at all, but was a collection of particles that were of equal mass to a proton but carried no charge whatsoever.Thusly, he named them neutrons. Chadwick's discovery earned him the Nobel Prize in physics in 1935.

Chadwick's neutrons displacing protons