Он3, or H3O+, is a hydronium ion formed when water (H2O) molecules gain an extra proton (H+). This ion plays a crucial role in acid-base chemistry, as it is responsible for the acidic properties of solutions. When an acid dissolves in water, it releases protons, which combine with water molecules to create hydronium ions. In aqueous solutions, the concentration of hydronium ions determines the pH level. A higher concentration of H3O+ indicates a lower pH, meaning the solution is more acidic. Understanding H3O+ is essential for studying chemical reactions, biological processes, and environmental science.