An atom is neutral when it has equal numbers of protons and electrons. Add or remove an electron, and the balance shifts. The atom becomes an ion, an atom or molecule with a net electrical charge. Lose an electron, and you get a positively charged cation. Gain an electron, and you get a negatively charged anion. Ions are everywhere: in seawater, in your blood, in the air, in rocks.

Ions form when atoms gain or lose electrons to achieve a stable electron configuration, usually a full outer shell. Metals tend to lose electrons and form cations. Nonmetals tend to gain electrons and form anions. The charge is determined by the number of electrons gained or lost. Sodium loses one electron to become Na+. Chlorine gains one to become Cl-. Magnesium loses two to become Mg2+. Oxygen gains two to become O2-.

Polyatomic ions are groups of atoms with a net charge. Nitrate (NO3-), sulfate (SO4^2-), phosphate (PO4^3-), and ammonium (NH4+) are common examples. They behave as single units in chemical reactions. They are essential in biology, industry, and environmental chemistry.

Ions conduct electricity in solution. They are the basis of electrolytes, which are essential for nerve function, muscle contraction, and fluid balance. They form ionic bonds, creating salts and crystals. They are involved in acid-base chemistry, redox reactions, and precipitation. The ion is a fundamental concept in chemistry.