Hydronium

Hydronium (also called oxonium) is an ion (a chemical compound with electric charge) with the chemical formula H₃O⁺. It is the result of adding a hydrogen ion to a water molecule (making it the conjugate acid of water). It is the simplest oxonium ion.

Structure of the hydronium ion, electrical charge not shown.

Hydronium ions are important in the chemistry of acids and bases. Because of the leveling effect, hydronium is the most acidic chemical species that can exist in water. Stronger acids will react with water to make hydronium. The pH of an acid in water is related to the concentration of hydronium in the solution.

Compounds

Solid salts containing hydronium are rare, but do exist for some very strong acids. They are sometimes called acid monohydrates, as they are the result of adding one equivalent of water to one equivalent of acid.^[1] Hydronium salts of perchloric acid,^[1] carborane acids,^[2] and fluoroantimonic acid ^[3] have been studied in crystal form.

The oxygen atom in a hydronium ion still has one lone pair, so it is in theory able to be a Lewis base and react with extremely strong acids. No stable compounds or solutions of protonated hydronium H₄O²⁺ have been made, but it has been shown to exist as a reaction intermediate in extremely acidic conditions.^[4]