In chemistry, an anion is a negatively charged particle—also known as a negative ion—that forms when an atom or group of atoms gains one or more electrons.

This gain results in an imbalance between the number of protons (positive charges) and electrons (negative charges), leading to an excess of negative charge.

Anions typically originate from elements with high electronegativity, such as chlorine or oxygen, which have a strong tendency to attract electrons.

They can be monatomic, when formed from a single atom (e.g., chloride or oxide), or polyatomic, when made up of multiple atoms bonded together, acting as a single negatively charged unit. Common polyatomic anions include sulfate, nitrate, and phosphate.

From a chemical perspective, anions play a crucial role. They combine with cations (positively charged ions) to form ionic compounds, such as sodium chloride (NaCl), where Na⁺ is the cation and Cl⁻ the anion. In aqueous solutions, anions contribute to electrical conductivity, making them key components of electrolytes.