Glossary
electron
An electron is a tiny negatively charged particle that moves around the nucleus; a neutral atom has as many as protons.
An electron is a very light, negatively charged particle that moves in the space around an atom's nucleus rather than inside it. It carries one unit of negative charge, so in a neutral atom the number of electrons equals the number of protons (the atomic number), and the charges cancel. Electrons are the part of the atom that moves during chemistry: an atom that gains electrons becomes a negative ion, and one that loses electrons becomes positive. The electrons farthest from the nucleus — the valence electrons — are the ones involved in bonding and in the periodic table's groups. Because an electron is nearly two thousand times lighter than a proton, changing the electron count barely changes the atom's mass, which is why the mass number counts protons and neutrons only.
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.
- ion — An ion is an atom (or group) with a charge, made by losing or gaining electrons; a metal loses, a nonmetal gains.
- valence electron — Valence electrons are the electrons in an atom's outermost shell; they decide bonding and the element's group.
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)