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Chemistry · Glossary

What is Ion, cation, anion?

Also known as: ion · cation · anion

Definition 17.1 School Chemistry — Grades 1 to 12 · Chapter 17 — Ions and Ionic Solutions

An ion is an atom, or a group of atoms, that has lost or gained one or more electrons, and so carries an electric charge. An ion with a positive charge, having lost electrons, is a cation; an ion with a negative charge, having gained electrons, is an anion. The charge is written at the top right of the formula: NaX+\ce{Na+}, CuX2+\ce{Cu^{2+}}, ClX−\ce{Cl-}.

A sodium atom becomes a sodium ion by losing an electron; a chlorine atom becomes a chloride ion by gaining one. The protons do not change.
A sodium atom becomes a sodium ion by losing an electron; a chlorine atom becomes a chloride ion by gaining one. The protons do not change.

Examples

Example 17.2 (Sodium and chlorine)

A sodium atom, 11 protons and 11 electrons, loses one electron: it becomes the sodium ion NaX+\ce{Na+}, 11 protons and 10 electrons, charge +1+1:

Na→NaX++eX−.\ce{Na -> Na+ + e-}.

A chlorine atom, 17 protons and 17 electrons, gains one electron: it becomes the chloride ion ClX−\ce{Cl-}, 17 protons and 18 electrons:

Cl+eX−→ClX−.\ce{Cl + e- -> Cl-}.

Example 17.11 (Writing a precipitation)

Only the ions that form the solid are written; the others stay dissolved and take no part:

CuX2+(aq)+2 OHX−(aq)→Cu(OH)X2(s),AgX+(aq)+ClX−(aq)→AgCl(s).\ce{Cu^{2+}(aq) + 2OH-(aq) -> Cu(OH)2(s)}, \qquad \ce{Ag+(aq) + Cl-(aq) -> AgCl(s)}.
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