1. A positively charged ion formed when a hydrogen ion (H⁺) attaches to a water molecule (H₂O), represented as H₃O⁺.
Usage: chemistry; also called hydronium ion or oxonium ion

Examples

• In aqueous solutions, hydrogen ions exist primarily as hydronium ions rather than free protons.
• The concentration of hydronium ions determines the pH of a solution.
• When an acid dissolves in water, it donates a proton to form hydronium ions.
• The hydronium ion is essential to understanding acid-base chemistry.
• Pure water contains a small amount of hydronium ions from self-ionization.