What does anion mean?
An anion is a negatively charged ion — an atom or molecule that has gained one or more electrons, giving it more electrons than protons. Anions are fundamental to chemistry: they form when substances dissolve or react, they carry electric current through solutions and molten salts, and they pair with positively charged ions (cations) to build compounds such as sodium chloride. The name comes from the Greek word for 'going up,' chosen by nineteenth-century scientists because, during electrolysis, anions migrate toward the positively charged electrode called the anode. Common examples include chloride, sulfate, nitrate, and hydroxide. Beyond the laboratory, the term appears in medicine, where the 'anion gap' is a standard blood-test measure used to evaluate acid-base balance, and in environmental science, where anions such as nitrate are monitored in water quality. Though strictly scientific in register, 'anion' is a precise, useful word for anyone describing how matter behaves at the atomic scale.
nounA negatively charged ion, formed when an atom or molecule gains one or more electrons; in electrolysis it migrates toward the anode.
- A negatively charged atom or molecule that has gained one or more electrons, which migrates to the anode during electrolysis.
"Chloride is a common anion found dissolved in seawater."
"In a solution of sodium chloride, the chloride ions are anions attracted to the positive electrode."
"Doctors often check the anion gap in a blood test to help diagnose acid-base disorders."
Regular plural; used freely in scientific writing.
"Both nitrate and phosphate are anions found in fertilizers."
The word 'anion' literally means 'the thing that goes up' — because in early electrolysis experiments it was the particle that climbed toward the positively charged electrode.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of anion
The word 'anion' was coined in the early nineteenth century from Greek 'ana-' meaning 'up' or 'back' plus '-ion', meaning 'that which goes'. It was introduced alongside 'cation' (from Greek 'kata-', 'down') by English scientists studying electrolysis, including Michael Faraday's circle, to describe ions moving toward the anode versus the cathode. Both terms derive ultimately from the same Greek root 'ienai', 'to go', which also gives us the general word 'ion'.
Related word forms
How anion is actually used
A technical term used chiefly in chemistry, physics, and medicine (e.g., 'anion gap' in clinical biochemistry). Neutral register; not used colloquially outside scientific contexts.
Easily confused with anion
An anion carries a negative charge and moves toward the anode, while a cation carries a positive charge and moves toward the cathode.