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The hydrogen ion, commonly denoted H+, is the positively charged species formed when a hydrogen atom loses its electron. In many environments, especially aqueous solutions, a bare proton does not exist freely; it is rapidly solvated by surrounding molecules, most prominently forming hydronium ions (H3O+) in water. Beyond H3O+, a family of hydrated proton forms are proposed, including structures such as Zundel (H5O2+) and Eigen (H9O4+), which describe how protons are stabilized by networks of hydrogen-bonded molecules.

In chemistry, the hydrogen ion is the central actor in acid–base reactions. Acids donate protons to bases, and the extent of proton transfer defines the strength of an acid, typically expressed by dissociation constants (Ka) and pKa values. The pH scale reflects the activity of hydrogen ions (in water, equivalently H3O+) and governs the direction of acid–base equilibria. Proton transfer is a common step in many reaction mechanisms, catalysis, and biological processes.

The hydrogen ion is remarkably mobile in solution. In water, proton transport occurs far more rapidly than for many other ions, via the Grotthuss mechanism, where protons effectively hop between hydrogen-bonded molecules. This high mobility contributes to strong proton conduction in acidic media and underpins the role of protons in bioenergetics, such as the proton-motive force used to drive ATP synthesis across membranes.

Outside water, the behavior of protons depends on the solvent and environment. In non-aqueous solvents and superacids, protons can exist as various solvated forms with different stabilities and reactivities. The hydrogen ion remains a fundamental concept across chemistry and biology, serving as the simplest and most ubiquitous example of a positively charged species involved in proton transfer.