Glossary
atomic number
The atomic number is the number of protons in an atom's nucleus; it also equals the electrons in a neutral atom.
The atomic number is the count of protons in an atom's nucleus, and it is the small whole number printed on each element's box in the periodic table. It is the element's identity: every atom of a given element has the same atomic number, and a different element has a different one. In a neutral atom the atomic number is also the number of electrons, because the positive protons and negative electrons must balance. The common error is to read the bigger number — the mass number — as the proton count. The atomic number never changes in an ordinary chemical reaction; it changes only in a nuclear reaction, which is why it names the element and not a particular form of it.
Taught in
Related terms
- atom — An atom is the smallest unit of an element, built from a nucleus of protons and neutrons with electrons around it.
- mass number — The mass number is the number of protons plus neutrons in an atom: neutrons = mass number − atomic number.
- ion — An ion is an atom (or group) with a charge, made by losing or gaining electrons; a metal loses, a nonmetal gains.
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)