Hydronium

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Hydroxonium (also called in short oxonium, and improperly hydronium) is a positively-charged ion that forms from water in the presence of hydrogen ions. The terminology (hydr)oxonium derives its IUPAC ethymology from the central oxygen (coordinating donor) atom, in agreement with ammonium NH₄⁺ (central coordinating donor atom N), phosphonium PH₄⁺ (coordinating atom P), stibonium SbH₄⁺ and so forth. This IUPAC rule holds both in inorganic and organic chemistry. In inorganic analytical chemistry the term hydroxonium can be abbreviated to oxonium, without ambiguity. The formula for (hydr)oxonium cation is H₃O⁺.

Oxonium is present whenever an acid is dissolved in water; the hydrogen ions (H⁺ or protons, i.e: IUPAC suggests the more general term hydrons instead, for a natural mixture of protons, deuterons, tritons) released by the acid do not exist in a free state (i.e: normally 3 water molecules are coordinated to an oxonium cation). Water will also spontaneously autodissociate into oxonium and hydroxide ions, in the following reaction:

2 H₂O TWO arrows of opposite versus does indicate chemical equilibrium, conversely ONE arrow indicates hybridization or mesomerism H₃O⁺ + OH⁻

The alkalinity of the OH⁻ ion released in this reaction balances oxonium's acidity, and water remains neutral at a pH of 7.0, i.e., at a oxonium ion concentration of 1 × 10^-7.0 mol/l at 298 K.