Definition · Plain-language
Ion
An ion is an atom or group of atoms that carries an electric charge because it has gained or lost electrons.
The step most authors miss
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A charged atom or molecule
A neutral atom has equal numbers of positively charged protons and negatively charged electrons, so its overall charge is zero. An ion forms when that balance is upset by gaining or losing electrons — the number of protons stays fixed, but the electron count changes. If an atom loses electrons it has more protons than electrons and becomes positively charged; if it gains electrons it has more electrons than protons and becomes negatively charged. Ions can be single atoms or groups of atoms acting as a unit, such as the hydroxide ion.
Cations and anions
Ions are named by the sign of their charge. A cation is a positively charged ion, formed by losing electrons; metals typically form cations, such as the sodium ion Na⁺ and the magnesium ion Mg²⁺. An anion is a negatively charged ion, formed by gaining electrons; non-metals typically form anions, such as the chloride ion Cl⁻ and the oxide ion O²⁻. A polyatomic ion is a group of bonded atoms that carries an overall charge, such as the sulfate ion or the ammonium ion.
Why ions matter
Ions are central to a great deal of chemistry and biology. When an ionic compound such as salt dissolves in water or melts, its ions become free to move and can carry an electric current, which is why salt water conducts electricity. Ions in the body — sodium, potassium, calcium — carry the electrical signals of nerves and muscles. In chemistry, the attraction between oppositely charged ions forms ionic bonds and builds the crystal lattices of salts. Reactions in solution are very often reactions between ions.
Key facts
At a glance
- Definition: an atom or molecule with an electric charge
- Formed by: gaining or losing electrons (not protons)
- Cation: positive ion — formed by losing electrons
- Anion: negative ion — formed by gaining electrons
- Polyatomic: a charged group of bonded atoms (e.g. sulfate)
- Why it matters: conduction, ionic bonds, nerve signals
Common misconceptions
What people often get wrong
Often heard: An ion forms when an atom gains or loses protons.
Actually: Ions form by gaining or losing electrons. The number of protons defines which element it is and does not change; altering protons would change the element itself, which is a nuclear process, not ionisation.
Often heard: A cation is negative and an anion is positive.
Actually: It is the other way round. A cation is a positive ion (formed by losing electrons), and an anion is a negative ion (formed by gaining electrons). A useful cue: the "t" in cation can look like a plus sign.
Often heard: An ion can only be a single atom.
Actually: Ions can also be groups of bonded atoms carrying an overall charge, called polyatomic ions — such as sulfate, nitrate or ammonium. These behave as a single charged unit in reactions.
