anion

IPA/ˈænaɪən/
KK[ˈænˌaɪɔn]IPA/ˈænaɪən/

anion — noun

  • anionsingular
  • anionsplural

1. an atom or small group of atoms that carries a negative electric charge because

1.nounC2
Definition

an atom or small group of atoms that carries a negative electric charge because it has gained one or more extra electrons.

Example

In a salt solution, chloride anions move toward the positive electrode during electrolysis.

domain context: anions in electrolysis

When a chlorine atom picks up one extra electron, it becomes an anion with a negative charge.

mechanism: gaining an electron forms an anion

Common collocations
  • chloride anion
  • hydroxide anion
  • sulfate anion
  • anion exchange
  • form an anion
Synonyms
  • negative ion

    everyday wording for the same particle; preferred outside specialist writing.

Antonyms
  • cation

    the matching term for a positively charged ion.

Grammar Patterns

anion of [element/compound]

Usage Note

Almost always used in chemistry contexts. Frequently paired with its opposite, 'cation' (positive ion), and named after the element or group involved (e.g. chloride anion, sulfate anion).

Common Mistakes

Sodium becomes an anion when it loses an electron.
Sodium becomes a cation when it loses an electron.
atoms that lose electrons form cations (positive); atoms that gain electrons form anions (negative).
The anion moves to the negative electrode.
The anion moves to the positive electrode (the anode).
opposite charges attract, so the negatively charged anion is pulled toward the positive side.

Word family

  • anionic
    adjectiveC2
  • cation
    nounC2
  • ion
    nounC1
  • ionic
    adjectiveC1
  • ionize
    verbC2

Synonym discussion

Sense: noun/1

All four words belong to chemistry, but they sit at different levels of detail. 'Ion' is the umbrella term — any atom or group of atoms that carries an electric charge. 'Anion' narrows that to ions with a negative charge, while 'cation' refers to ones with a positive charge; the two always come as a contrasting pair. 'Electrolyte' is broader still: it names a substance (such as a salt or acid dissolved in water) that splits into ions and lets electricity pass through. So in the sentence 'When salt dissolves, the electrolyte separates into ions — sodium cations and chloride anions,' each term has a clear, non-overlapping job: electrolyte = the substance, ion = the charged piece, anion / cation = which sign of charge it carries.

anion ↗cation ↗ion ↗electrolyte

Common collocations

Nouns

  • anion exchange
  • anion concentration
  • chloride anion
  • hydroxide anion
  • sulfate anion

Phrases

  • form an anion
  • produce an anion
  • anions and cations

Prepositions

  • anion of
  • anion in
  • anion with

Etymology

GreekEnglish

Coined in the 1830s by the English scientist Michael Faraday, who needed new vocabulary for his work on electrolysis. He took the Greek word 'anion', meaning 'a thing going up', because in his experiments these particles travelled upward through the solution toward the upper electrode. The matching word 'cation' (a thing going down) was coined at the same time. Both words entered scientific English together and have kept their original meanings since.