Anion Definition and Examples

An anion is an ionic species having a negative charge. The chemical species may be a single atom or a group of atoms. An anion is attracted to the anode in electrolysis. Anions are typically larger than cations (positively charged ions) because they have extra electrons around them.

The word anion [AN-aye-un] was proposed by English polymath the Rev. William Whewell in 1834, from the Greek anion, or "thing going up," referring to the movement of anions during electrolysis. Physicist Michael Faraday was the first person to use the term in a publication.

Examples

Here are some common anions:

• Free chloride in an aqueous table salt (NaCl) solution: Cl^-
• Singlet oxygen: O^-
• Superoxide: O^2-
• Hydroxide ion: OH^-
• Sulfate: SO₄^2-
• Al(OH)₄^-

Notation

When naming a chemical compound, the cation is given first, followed by the anion. For example, the compound sodium chloride is written NaCl, where Na⁺ is the cation and Cl^- is the anion.

The net electrical charge of an anion is denoted using a superscript after the chemical species symbol. For example, the phosphate ion PO₄^3- has a charge of 3-.

Since many elements display a range of valences, determining the anion and cation in a chemical formula isn't always clear-cut. In general, the difference in electronegativity can be used to identify the cation and anion in a formula. The more electronegative species in a chemical bond is the anion.​