What is a neutron in an atom?

A neutron is a subatomic particle found in the nucleus of an atom. It has about the same mass as a proton but has no electric charge, and it contributes to the atom's total mass and isotope type.

How do you find the number of neutrons in an atom?

To find the number of neutrons, subtract the atomic number (number of protons) from the mass number (total number of protons and neutrons). For example, if an atom has a mass number of 23 and an atomic number of 11, it has 23 - 11 = 12 neutrons.

Why are neutrons important?

Neutrons help hold the nucleus together by balancing the repulsion between positively charged protons. They also determine which isotope of an element an atom is and play a key role in nuclear reactions such as fission.

Do neutrons move or stay still in the nucleus?

Neutrons are not fixed in place; they move inside the nucleus along with protons. However, they are held very tightly by the strong nuclear force, so they stay inside the nucleus under normal conditions.

What Is a Neutron? Definition and Examples

Kid-friendly explanation

Every atom is like a tiny solar system. In the middle is the nucleus, which holds protons and neutrons. Protons are like positively charged marbles, and neutrons are marbles with no charge at all. Neutrons add weight to the atom and help hold the nucleus together!

Worked examples

Example 1 (middle school level): Counting neutrons in an atom

Suppose an atom of carbon has a mass number of 12 and an atomic number of 6. The atomic number (6) tells how many protons there are. The mass number (12) tells the total number of protons and neutrons.

To find the number of neutrons, subtract the number of protons from the mass number:

Number of neutrons = mass number − number of protons = 12 − 6 = 6.

So this atom of carbon has 6 neutrons.

Example 2 (high school level): Comparing isotopes using neutrons

Consider two isotopes of chlorine: chlorine-35 and chlorine-37. Both are atoms of chlorine, so each has 17 protons (atomic number 17). The mass numbers are 35 and 37.

For chlorine-35: neutrons = 35 − 17 = 18.

For chlorine-37: neutrons = 37 − 17 = 20.

These atoms have the same number of protons (so they are both chlorine) but different numbers of neutrons, which makes them different isotopes with slightly different masses.

Going deeper

Neutrons, together with protons, are called nucleons because they live in the nucleus of the atom. The total number of nucleons is the mass number, which is why changing the number of neutrons changes the mass number but not the element itself.

Atoms of the same element that have different numbers of neutrons are called isotopes. For example, carbon-12 and carbon-14 are both carbon, but carbon-14 has 2 more neutrons, which makes it heavier and radioactive, so it can be used in carbon dating.

At a more advanced level, neutrons are important in nuclear reactions. In nuclear fission, a neutron can hit a large nucleus, cause it to split into smaller nuclei, release energy, and often release more neutrons that can continue the chain reaction.

Common mistakes and how to fix them

Mistake: Thinking neutrons have a negative charge because the word sounds like "neutral" or "negative".
Fix: Neutrons have no electric charge at all; they are neutral, not negative. Electrons are the negatively charged particles.
Mistake: Using the atomic number to find neutrons directly, for example saying an atom with atomic number 8 has 8 neutrons.
Fix: The atomic number tells the number of protons. To find neutrons, subtract the atomic number from the mass number: neutrons = mass number − atomic number.
Mistake: Believing that changing the number of neutrons changes the element itself (for example, thinking carbon with extra neutrons becomes nitrogen).
Fix: The element is set by the number of protons. Changing neutrons only changes the isotope and the mass, not the element's identity.

How neutrons connect to everyday life

Neutrons are too small to see, but their effects show up in many parts of daily life. Smoke detectors, some medical imaging and cancer treatments, and nuclear power plants all rely on how neutrons behave in atoms and in nuclear reactions. Even the dating of ancient fossils and artifacts uses isotopes that differ only in their numbers of neutrons.

Related terms to explore

These chemistry ideas connect closely to how atoms and particles behave:

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What is a neutron?