Glossary
ion
An ion is an atom or group of atoms with a charge, made by losing electrons (positive cation) or gaining them (negative anion).
An ion is an atom or a group of atoms carrying a charge, and the charge comes from electrons. Electrons are the part of an atom that can be gained or lost cheaply, so an atom that loses one has more protons than electrons and becomes a positive ion — a cation. An atom that gains one has more electrons than protons and becomes negative — an anion. Sodium loses its single valence electron to become Na+, and chlorine gains one to become Cl-, and the two then attract. The charge is written as a superscript, and the number comes from how many electrons moved. A group of atoms can carry a charge too: sulfate and nitrate are polyatomic ions, and they behave as units. The naming follows the charge — a metal that forms more than one ion takes a Roman numeral, as in iron(II) and iron(III) — which is why the same metal can appear in two different formulas.
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Related terms
- ionic bond — An ionic bond is the attraction between two oppositely charged ions, formed when one atom gives electrons to another — a metal and a nonmetal, usually.
- atomic number — An element's atomic number is the number of protons in its nucleus, and it is what decides which element the atom is.
- isotope — Isotopes are atoms of the same element with different numbers of neutrons, so they share chemistry but differ in mass.
Source: NIST periodic table and reference data for the atomic masses and half-life figures, NIH PubChem for the substance data, and NIST Special Publication 811 for the unit conventions — the same sources the chemistry guide cites. All three are US federal government material, not subject to copyright in the United States. Every definition, example and calculation above was written for this course; no dictionary or textbook text is reproduced. (NIST and PubChem accessed 2026-10-26; definitions written 2026-10-27)