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Ions are atoms or molecules that carry a net electric charge because they have gained or lost electrons. This occurs when atoms satisfy their electron configuration by removing electrons (forming cations, positive) or adding electrons (forming anions, negative). Monatomic ions are single atoms such as Na+, Cl-, and Ca2+. Polyatomic ions are groups of atoms with an overall charge, such as NH4+, NO3-, and SO4^2-.

Ions form when chemical reactions or dissolution occur. In solids, ions attract to form crystalline lattices held together by electrostatic forces. In solution, ions are surrounded by solvent molecules in a process called hydration; their mobility enables electrical conduction.

Ion charges influence chemical behavior: cations are attracted to negatively charged electrodes; anions to positive electrodes. Ionic compounds such as sodium chloride consist of alternating cations and anions in a lattice. When dissolved, these ions disperse and can react or conduct electricity.

Ions play a central role in biology and environmental science. Sodium, potassium, calcium, and chloride ions regulate nerve signaling and muscle function; hydrogen ions determine pH. Ions also participate in redox chemistry, catalysis, and energy storage in batteries and fuel cells.