Hydronium

Hydronium, or the hydronium ion, is the positively charged species with formula H3O+. It forms when a proton adds to a water molecule, producing H3O+. In liquid water, free protons do not exist as isolated H+ ions; proton transfer occurs through networks of hydrogen-bonded water molecules, and discrete H+ ions do not persist.

In aqueous solutions, protons are transferred via water autoprotolysis: 2 H2O ⇌ H3O+ + OH−. At 25 °C, the equilibrium constant Kw is about 1.0 × 10−14, so neutral water has [H3O+] = [OH−] ≈ 1.0 × 10−7 M. The hydronium ion is the conjugate acid of water and is the species most directly associated with acidity in water. The pH of an aqueous solution is defined by pH = −log10[H3O+].

Structure and hydration: The H3O+ ion has a roughly trigonal pyramidal geometry, with the positive charge distributed over the O–H bonds. In bulk water, hydronium exists as a dynamically hydrated proton, and larger protonated-water clusters can form. Commonly discussed species include the Zundel cation H5O2+ and the Eigen cation H9O4+, which represent ordered arrangements of excess proton hydration in different environments.

Role in chemistry: Hydronium is central to acid-base reactions in aqueous media. Protons transfer between water molecules via hydrogen-bond networks, described by the Grotthuss mechanism, affecting reaction rates, equilibria, and proton conduction. In electrochemistry and catalysis, hydronium activity influences acidity, catalytic cycles, and transport properties.