WHAT IS AN ANION? - 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. 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-.

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What Is an Anion?

Anion Definition and Examples

Chemistry Basics: What Is an Anion?

by Anne Marie Helmenstine, Ph.D.

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-ahy-uh n] was proposed by English polymath Rev. William Whewell in 1834, from Greek anion "thing going up", referring to the movement of anions during electrolysis.

Physicist Michael Faraday was the first person to use the term anion in a publication.

Anion Examples

·       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^-

Anion 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 clearcut.

In general, the difference in electronegativity may be used to identify the cation and anion in a formula.

The more electronegative species in a chemical bond is the anion.​ 

Table of Common Anions

Simple Anions	Formula
Hydride	H-
Oxide	O2-
Fluoride	F-
Sulfide	S2-
Chloride	Cl-
Nitride	N3-
Bromide	Br-
Iodide	I-
Oxoanions	Formula
Arsenate	AsO43-
Phosphate	PO43-
Arsenite	AsO33-
Hydrogen Phosphate	HPO42-
Dihydrogen Phosphate	H2PO4-
Sulfate	SO42-
Nitrate	NO3-
Hydrogen Sulfate	HSO4-
Nitrite	NO2-
Thiosulfate	S2O32-
Sulfite	SO32-
Perchlorate	ClO4-
Iodate	IO3-
Chlorate	ClO3-
Bromate	BrO3-
Chlorite	ClO2-
Hypochlorite	OCl-
Hypobromite	OBr-
Carbonate	CO32-
Chromate	CrO42-
Hydrogen Carbonate or Bicarbonate	HCO3-
Dichromate	Cr2O72-
Anions from Organic Acids	Formula
Acetate	CH3COO-
Formate	HCOO-
Other Anions	Formula
Cyanide	CN-
Amide	NH2-
Cyanate	OCN-
Peroxide	O22-
Thiocyanate	SCN-
Oxalate	C2O42-
Hydroxide	OH-
Permanganate	MnO4-

Anne Marie Helmenstine, Ph.D.

·   Ph.D. in biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge National Laboratory.

·   Science educator with experience teaching chemistry, biology, astronomy, and physics at the high school, college, and graduate levels.

·   ThoughtCo and About Education chemistry expert since 2001.

·   Widely-published graphic artist, responsible for printable periodic tables and other illustrations used in science.

Experience

Anne Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education since 2001, and other sciences since 2013. She taught chemistry, biology, astronomy, and physics at the high school, college, and graduate levels. She has worked as a research scientist and also abstracting and indexing diverse scientific literature for the Department of Energy.

In addition to her work as a science writer, Dr. Helmenstine currently serves as a scientific consultant, specializing in problems requiring an interdisciplinary approach. Previously, she worked as a research scientist and college professor. 

Education

Dr. Helmenstine holds a Ph.D. in biomedical sciences from the University of Tennessee at Knoxville and a B.A. in physics and mathematics with a minor in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive chemical detection and medical diagnostic tests.

Anne Marie Helmenstine, Ph.D.​

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