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An anion is an ion with a net negative charge. It may be a single atom or a group of atoms, and the charge comes from having more electrons than protons. Calling an anion “electron-loving” is a catchy metaphor, not its definition: electron affinity is a separate way of describing the energy involved when an electron attaches to a neutral species.
What is an anion?
The International Union of Pure and Applied Chemistry (IUPAC) defines an anion as “A monoatomic or polyatomic species having one or more elementary charges of the electron.” In plain language, an anion is a negatively charged ion. It can be one atom, such as chloride, or a group of atoms, such as sulfate.
The definition describes the species’ charge; it does not require one universal way for the species to form. In the familiar introductory case, an atom gains one or more electrons. Because electrons carry negative charge, this gives the atom more electrons than protons. The proton count ordinarily remains unchanged during this electron-transfer process.
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An atom is electrically neutral when its positive protons and negative electrons balance. If it gains an electron without changing its proton count, negative charge outweighs positive charge and the atom becomes a negative ion. An atom that gains two electrons has a net charge of 2−; one that gains a single electron has a net charge of 1−.
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This is a useful introductory account of electron transfer, not a claim that every anion must be formed by adding electrons to an isolated neutral atom. IUPAC’s definition covers monoatomic and polyatomic species and focuses on their negative charge.
Anion vs. cation: what is the difference?
| Ion | Net charge | Introductory electron-balance description |
|---|---|---|
| Anion | Negative | Gains electrons, leaving more electrons than protons |
| Cation | Positive | Loses electrons, leaving fewer electrons than protons |
The terms identify opposite charge signs. The gain-and-loss descriptions are a straightforward way to understand many familiar ions: electron gain adds negative charge, while electron loss removes some negative charge. The number of protons does not change in ordinary ion formation by electron transfer.
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Does gaining an electron release energy?
It depends on how the energy is described and on the process being discussed. IUPAC defines electron affinity as the energy required to detach an electron from a singly charged negative ion. It also gives the equivalent attachment description: the energy released when an electron attaches to a neutral atom or molecule. These descriptions refer to opposite directions of the same change, so state whether the discussion concerns attachment or detachment and whether energy is released or required.
Electron affinity is an energy quantity, not another name for an anion and not a measure of an atom’s personality. Saying an atom “wants” an electron can obscure the actual question: which species is involved, what process is taking place, and how is the energy reported?
When should you say “negative ion” instead of “anion”?
For ordinary solution chemistry, “anion” is standard terminology. IUPAC makes a narrower recommendation for an atom, radical, molecule, or molecular moiety in the vapour phase after it gains electrons: use “negative ion.” IUPAC notes that “anion” carries solution-chemistry connotations in this context. This is a phase-specific terminology distinction, not a general prohibition on calling ions in solution anions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Key points to remember
- An anion has a net negative charge; a cation has a net positive charge.
- An anion can consist of one atom or multiple atoms.
- Electron gain is a common way to explain negative-ion formation, but the definition is about charge rather than a single formation pathway.
- Electron affinity describes energy in electron attachment or detachment; name the direction and energy convention.
Sources: IUPAC, “anion,” Compendium of Chemical Terminology (Gold Book), 5th edition (2025); IUPAC, “electron affinity”; IUPAC, “negative ion”; Khan Academy, ions and ionic compounds.
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