Glossary
atom
An atom is the smallest unit of an element, built from a nucleus of protons and neutrons with electrons around it.
An atom is the smallest particle of an element that still is that element. It has a tiny, dense nucleus holding protons (positive) and neutrons (no charge), and a cloud of electrons (negative) that move around the nucleus. The number of protons fixes which element it is; the number of neutrons can vary, giving heavier or lighter forms of the same element. A neutral atom has equal numbers of protons and electrons, so its charges cancel. Almost all of an atom's mass sits in the nucleus, because a proton and a neutron are far heavier than an electron. Atoms join into compounds by gaining, losing or sharing electrons — the basis of bonding; breaking a compound back into its elements takes a chemical change, and changing an atom itself takes a nuclear reaction.
Taught in
Related terms
- atomic number — The atomic number is the number of protons in an atom's nucleus; it also equals the electrons in a neutral atom.
- 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)