In chemistry, an ion is a particle (an atom or group of atoms) that carries an electrical charge due to the loss or gain of one or more electrons. When an atom loses electrons, it becomes a positive ion, known as a cation. Conversely, if it gains electrons, it becomes a negative ion, called an anion.

Ions form because atoms tend to achieve a more stable electron configuration, often resembling that of noble gases. For example, sodium (Na) loses one electron to become Na⁺, a cation, while chlorine (Cl) gains one electron to become Cl⁻, an anion.

An ion's charge is indicated by a plus or minus sign and a number showing how many electrons have been lost or gained. Ions can be monatomic (like Ca²⁺) or polyatomic (like NO₃⁻), meaning they consist of multiple atoms bonded together that act as a single charged unit.

Ions play a crucial role in chemical processes: they form salts (such as sodium chloride, NaCl), conduct electricity in aqueous solutions (electrolytes), and are involved in many biological and industrial reactions. In solution, they move under the influence of electric fields and are responsible for ionic conduction.