Glossary
neutron
A neutron is an uncharged particle in the nucleus; neutrons = mass number − atomic number.
A neutron is a particle in an atom's nucleus that carries no charge — it is electrically neutral, which is where its name comes from. It weighs about the same as a proton, so together the protons and neutrons make up almost all of an atom's mass. You find the neutron count by subtracting: neutrons = mass number − atomic number. Hydrogen with atomic number 1 and mass number 1 has no neutrons at all, while a sodium atom with atomic number 11 and mass number 23 has twelve. Atoms of one element with different numbers of neutrons are heavier or lighter forms of that element. The common error is to treat the mass number as the neutron count, or to add the atomic number to the mass number instead of subtracting.
Taught in
Related terms
- proton — A proton is a positively charged particle in the nucleus; its count is the atomic number and fixes the element.
- mass number — The mass number is the number of protons plus neutrons in an atom: neutrons = mass number − atomic number.
- atom — An atom is the smallest unit of an element, built from a nucleus of protons and neutrons with electrons around it.
Source: OpenStax Chemistry 1e and Chemistry: Atoms First 1e, first editions (CC BY 4.0). Each definition restates the Chemistry guide in this exam, which carries the per-source citations on its own page. (OpenStax 1e accessed 2026-10-08)