A chlorine atom has 17 protons and 17 electrons. It needs one more electron to fill its outer shell. When it gains that electron, it becomes a chloride ion, with 17 protons and 18 electrons. The extra electron gives it a net negative charge. A negatively charged ion is called an anion. The name comes from the Greek word meaning going up, because anions migrate toward the anode in electrolysis.

Anions form when atoms gain electrons. Nonmetals tend to form anions because they have nearly full outer shells and gain electrons more easily than they lose them. Oxygen forms the oxide anion, O2-. Sulfur forms the sulfide anion, S2-. Nitrogen forms the nitride anion, N3-. The charge depends on how many electrons the atom needs to reach a stable configuration.

Polyatomic anions are groups of atoms with a net negative charge. Nitrate (NO3-), sulfate (SO4^2-), carbonate (CO3^2-), and phosphate (PO4^3-) are common examples. They are stable units that carry a charge and participate in reactions as a group. They are essential in biology, industry, and environmental chemistry.

Anions are everywhere. Table salt is sodium cation paired with chloride anion. Baking soda is sodium cation paired with bicarbonate anion. The balance of cations and anions in a solution determines its electrical conductivity and its pH. In the human body, the balance of anions and cations maintains fluid balance, nerve function, and acid-base homeostasis.