hydrogenion

Hydrogen ion, or hydrogenion, is the positively charged ion formed when a hydrogen atom loses its electron. In the gas phase, this species is simply a bare proton, represented as H+. In aqueous solution, free protons are not observed; they are strongly solvated by water to form hydronium ions, typically written as H3O+.

Hydrogen ions are central to acid–base chemistry. Arrhenius acids dissociate to yield H+ in water; Brønsted–Lowry acids donate protons; the concept of proton transfer underpins many biochemical and industrial processes. In many contexts, the behavior of H+ is described through its interactions with bases, solvents, and an established pH scale.

In water and most biological and chemical systems, the concept of a free H+ is replaced by hydrated forms. The autoprotolysis of water gives rise to hydronium and hydroxide ions: 2 H2O ⇌ H3O+ + OH−, a balance that defines pH via the activity of H3O+ and the relation pH = −log10(activity of H3O+).

Free hydrogen ions are highly reactive and do not exist as isolated entities in condensed phases; their behavior is governed by solvation, proton affinity, and donor–acceptor interactions. In biological systems, proton transfer and proton gradients across membranes drive energy conversion, signaling, and transport processes.

Name and terminology: The term "hydrogen ion" is common in chemistry, while "proton" is widely used in physics and biochemistry. In aqueous media, "hydronium" refers to H3O+, and hydrated forms such as Zundel and Eigen complexes are discussed in more advanced contexts.