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.

Structure

Structural formula of the Eigen cation, where three water molecules are attached to a hydronium cation by strong hydrogen bonds.

Hydronium is often written as H₃O⁺, but this is a very simplified formula. The full behavior of the hydrogen ion in water is not completely understood by scientists. Hydronium participates in hydrogen bonding like water, and tends to form clusters of surrounding water molecules. Because of the extra charge, these bonds are even stronger than the bonds between water molecules.

Two reasonably well-defined hydronium cations are the Zundel cation H₅O+2 and Eigen cation H₉O+4.^[1]

In the Zundel cation, a symmetric hydrogen bond shares the hydrogen ion equally between two water molecules. The Zundel cation forms four more hydrogen bonds from the other four hydrogen atoms, so it is sometimes written H₁₃O+6 or (H₂O)₄H₅O+2 when dissolved in water.^[2]

In the Eigen cation, a central H₃O⁺ forms a hydrogen bond through each of its hydrogen atoms, each attaching to a different water molecule.

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.^[3] Hydronium salts of perchloric acid,^[3] carborane acids,^[4] and fluoroantimonic acid ^[5] 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.^[6]