Neutrons: Definition & Concept

Instructor: Kelly Robson

Kelly has taught High School Science and Applied Communications. She holds an Education Specialist Degree in Ed. Leadership.

Neutrons are the big guys of the atom. This lesson explains the structure of the neutron, its properties, and some of its influences on the function of the atom.


Neutrons are the largest of the particles that make up the atom. The neutron is a baryon, which means it is a massive particle that is made up of 3 quarks (1 up quark and 2 down quarks). Quarks are what build all matter. You may have heard that atoms are the building blocks of all matter and this is true, but now scientists have figured out that quarks build all the parts that make up atoms.

This model of a neutron quark structure shows the 2 down quarks (d) and the 1 up quark (u).
Structure of a neutron

Neutrons are found in the nucleus of the atom. They are tightly packed with the protons. The protons and neutrons are held together in the nucleus with a force called the strong nuclear force. The nucleus is a very dense area that is found in the center of the atom.

Neutrons are found in the nucleus of the atom with protons.
Atomic Structure

Neutrons have a neutral charge or actually no charge at all. Its partner in the nucleus, the proton, does have a positive charge. The proton's positive charge is matched with the electron's negative charge to make a neutral atom. Even though the neutron does not impact the charge of the atom, it still has many properties that influence the atom, including the atom's level of radioactivity, but we will get to that later.


The neutron is 0.2% larger than the proton. Together, the neutron and proton make up 99.99% of all the mass of the atom. The neutron actually has the same mass as an electron and proton put together. The atomic mass of an atom is found by adding the number of protons and neutrons. Since the atomic number refers to the number of protons of an atom, we can use both the atomic mass and the atomic number to figure out the number of neutrons. When using the simple equation below, we are able to find the number of neutrons in an atom.

Atomic Mass - Atomic Number = Number of Neutrons

For instance, Carbon has an atomic mass of 12 and an atomic number of 6. So if we place these numbers in our equation, it will look like this…

12 (Atomic Mass) - 6 (Atomic Number) = 6 Neutrons.

Isotopes and Radioactivity

Atoms of the same element may have different numbers of neutrons, so when scientists refer to the atomic mass, they are referring to the average atomic mass. For instance, Carbon usually has 6 neutrons and 6 protons with an atomic mass of 12, but sometimes it is found with an atomic mass of 13 (6 protons and 7 neutrons). Carbon with an atomic number of 14 also exists but is rare. So, the atomic mass for Carbon is averaged to 12.011.

When atoms have a different amount of neutrons than their normal expected amount, they are called isotopes. So the example above with Carbon would make C-12 normal and C-13 and C-14 isotopes of Carbon.

Scientists have found ways of adding neutrons to the nucleus to make larger isotopes. Now, adding neutrons does not affect the charge of the atom since the neutron does not have a charge. However, it does increase the radioactivity of the atom. This could make very unstable atoms which can discharge high levels of energy.

When an atom loses a neutron, the process is called radioactive decay. C-14 is an isotope that often reverts to its more comfortable C-12 state. Since it is known how long it takes for this process to happen, archaeologists are able to use the information from the number of neutrons in the carbon to figure out how old something is even if it has been buried for a long time. This process is called carbon dating.

Archaeologists rely on information from neutrons to help them determine the age of something they may find on a dig.
Archaeologists at work

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