Hydron (chemistry)

Hydron (chemistry)

In chemistry, a hydron is the general name for a cationic form of atomic hydrogen H+: most commonly a "proton". However, hydron includes cations of hydrogen regardless of their isotopic composition: thus it refers collectively to protons (1H+), deuterons (2H+ or D+), and tritons (3H+ or T+). Unlike other ions, the hydron consists only of a bare atomic nucleus.

The hydron (a completely free or "naked" hydrogen atomic nucleus) is too reactive to occur in many liquids, even though it is sometimes visualized to do so by students of chemistry. A free hydron would react with a molecule of the liquid to form a more complicated cation. Examples are the hydronium ion in water-based acids, and H₂SbF₆+, the unstable cation of fluoroantimonic acid, the strongest superacid. For this reason, in such liquids including liquid acids, hydrons diffuse by contact from one complex cation to another, via the Grotthuss mechanism.

The hydrated form of the hydrogen cation, the hydronium (hydroxonium) ion H₃O+, is a key object of Arrhenius' definition of acid. Other hydrated forms, the Zundel cation H₅O+
2 which is formed from a proton and two water molecules, and the Eigen cation H₉O+
4, a hydronium ion and three water molecules, play an important role in "hydron hopping" according to the Grotthuss mechanism. The hydron itself is crucial in more general Brønsted–Lowry acid–base theory, which extends the concept of acid–base chemistry beyond aqueous solutions.

The negatively charged counterpart of the hydron is the hydride anion, H−.