Science, Tech, Math › Science Anion Definition and Examples An anion is an ionic species with a negative charge Share Flipboard Email Print Sodium chloride is written NaCl, where Na+ is the cation and Cl- is the anion. Fernando Trabanco Fotografía / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated January 07, 2020 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: O2-Hydroxide ion: OH-Sulfate: SO42-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 PO43- 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.