High Quality Content by WIKIPEDIA articles! In chemistry, a hydronium ion is the cation H3O+, a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated (bound to a solvent). The reality is far more complicated, and a proton is bound to several molecules of water, such that other descriptions such as H5O2+, H7O3+ and H9O4+ are increasingly accurate description of the environment of a proton in water. The ion H3O+ has been detected in the gas phase.