Definition · Plain-language
Cation and anion
A cation is a positively charged ion formed by losing electrons; an anion is a negatively charged ion formed by gaining electrons.
The step most authors miss
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Two kinds of charged ion
Both cations and anions are ions — atoms or groups of atoms with an electric charge — but they differ in the sign of that charge and how it arises. A cation carries a positive charge because it has lost one or more electrons, so its protons now outnumber its electrons. An anion carries a negative charge because it has gained electrons, so its electrons now outnumber its protons. The number of protons never changes; it is the gain or loss of electrons that creates the charge and decides which type of ion forms.
Which atoms form which
There is a broad pattern. Metals tend to lose their loosely held outer electrons, so they usually form cations: sodium becomes Na⁺, magnesium Mg²⁺, aluminium Al³⁺. Non-metals tend to gain electrons to complete their outer shell, so they usually form anions: chlorine becomes chloride Cl⁻, oxygen becomes oxide O²⁻. The size of the charge reflects how many electrons are lost or gained. Polyatomic ions, made of several bonded atoms, can be either — ammonium is a cation, while sulfate and nitrate are anions.
How they behave in compounds and cells
Cations and anions attract one another because opposite charges attract, and this attraction forms ionic bonds and builds the lattices of salts, where the total positive and negative charge always balances. In electrolysis, cations move to the negative electrode (the cathode) and anions move to the positive electrode (the anode) — a handy way to remember the names. In the body, cations such as sodium and potassium and anions such as chloride carry the electrical signals of nerves and keep fluids in balance.
Key facts
At a glance
- Cation: a positive ion, formed by losing electrons
- Anion: a negative ion, formed by gaining electrons
- Cations from: usually metals (Na⁺, Mg²⁺, Al³⁺)
- Anions from: usually non-metals (Cl⁻, O²⁻)
- In electrolysis: cations go to the cathode, anions to the anode
- In compounds: charges balance so the compound is neutral
Common misconceptions
What people often get wrong
Often heard: A cation is negatively charged.
Actually: A cation is positively charged, formed by losing electrons. An anion is the negatively charged ion, formed by gaining electrons. Confusing the two is one of the most common errors with ions.
Often heard: Cations and anions are completely different particles from ions.
Actually: Cations and anions are simply the two kinds of ion. "Ion" is the general term; cation and anion just name whether the charge is positive or negative.
Often heard: The charge on an ion comes from gaining or losing protons.
Actually: The charge comes from gaining or losing electrons. The proton count stays fixed and defines the element. A cation has lost electrons; an anion has gained them.
