H3O

H3O, more properly written H3O+, is the hydronium ion. It is the positively charged species formed when a proton (H+) associates with a water molecule (H2O) in aqueous solution. The simple representation H3O+ conveys the idea of a water molecule bearing an extra proton, but in reality the solvated proton rapidly exchanges and motion through the hydrogen-bond network creates a distribution of hydrated forms.

Formation and role in water self-ionization: In pure water, protons are not free; the autoprotolysis of water produces equal concentrations of hydronium and hydroxide ions: 2 H2O ⇌ H3O+ + OH−. The concentration of H3O+ defines the solution’s pH via pH = −log10[H3O+]. At room temperature, neutral water has [H3O+] = 1 × 10−7 M and Kw = [H3O+][OH−] ≈ 1.0 × 10−14. The hydronium ion is central to Brønsted acidity in aqueous systems, serving as the species that donates protons to bases.

Structure, solvation, and proton transport: H3O+ has a trigonal pyramidal geometry with three O–H bonds and a lone pair on the oxygen. In water, the proton is highly solvated and participates in rapid exchange with surrounding water molecules. Proton transport occurs mainly through hydrogen-bond networks by the Grotthuss mechanism, effectively moving the charge through the solution without the physical movement of a discrete proton across large distances.

Occurrence and terminology: H3O+ is ubiquitous in acidic aqueous solutions and is the reference species for aqueous acidity. In more detailed models of proton solvation, the proton exists as a family of hydrated forms (for example, H5O2+ and larger networks), with H3O+ as a convenient shorthand for the solvated proton.