Ion | Definition, Chemistry, Examples, & Facts | Britannica

Ask the Chatbot Games & Quizzes History & Society Science & Tech Biographies Animals & Nature Geography & Travel Arts & Culture ProCon Money Videos ion Introduction References & Edit History Related Topics Images & Videos Contents Science Chemistry CITE verifiedCite While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions. Select Citation Style MLA APA Chicago Manual of Style Copy Citation Share Share Share to social media Facebook X URL https://www.britannica.com/science/ion-physics Feedback External Websites Feedback Corrections? Updates? Omissions? Let us know if you have suggestions to improve this article (requires login). Feedback Type Select a type (Required) Factual Correction Spelling/Grammar Correction Link Correction Additional Information Other Your Feedback Submit Feedback Thank you for your feedback

Our editors will review what you’ve submitted and determine whether to revise the article.

External Websites

• Khan Academy - Introduction to ions
• MHCC Library Press - Ions
• Middle Tennessee State University - Department of Chemistry - Ions, Ionic Compounds, and Nomenclature
• National Center for Biotechnology Information - PubMed Central - Interacting Ions in Biophysics: Real is not Ideal
• Open Washington Pressbooks - Chem&121: Introduction to Chemistry - Ions and Ionic Compounds
• Chemistry LibreTexts - Ions

ion physics Ask Anything Homework Help Written and fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Nov. 18, 2025 •History Britannica AI Ask Anything Homework Help Table of Contents Table of Contents Ask Anything

ion, any atom or group of atoms that bears one or more positive or negative electrical charges. Positively charged ions are called cations; negatively charged ions, anions. Ions are formed by the addition of electrons to, or the removal of electrons from, neutral atoms or molecules or other ions; by combination of ions with other particles; or by rupture of a covalent bond between two atoms in such a way that both of the electrons of the bond are left in association with one of the formerly bonded atoms. Examples of these processes include the reaction of a sodium atom with a chlorine atom to form a sodium cation and a chloride anion; the addition of a hydrogen cation to an ammonia molecule to form an ammonium cation; and the dissociation of a water molecule to form a hydrogen cation and a hydroxide anion.

Key People: Dennis Robert Hoagland David Wineland Wolfgang Paul Pyotr Petrovich Lazarev (Show more) Related Topics: carbanion carbonium ion ionization isopoly anion lyonium ion (Show more) See all related content

Many crystalline substances are composed of ions held in regular geometric patterns by the attraction of the oppositely charged particles for each other. Ions migrate under the influence of an electrical field and are the conductors of electric current in electrolytic cells.

Keep Learning

• How do atoms become ions?
• Why are ions important in the human body?
• What is electrolysis and how does it use ions?
• How do ions form chemical bonds?
• What role do ions play in batteries?

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn.