Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes. Depending on the charge of the ion, two different classes can be distinguished:

Cation (positively charged)

When hydrogen loses its electron, the following cations can be formed:

• Hydron: general name referring to the positive ion of any hydrogen isotope (H⁺)
• Proton: ¹H⁺
• Deuteron: ²H⁺, D⁺
• Triton: ³H⁺, T⁺

In addition, the ions produced by the reaction of these cations with water as well as their hydrates are called hydrogen ions:

• Hydronium ion: H₃O⁺
• Zundel cation: H₅O₂⁺
• Eigen cation: H₉O₄⁺

The latter two play an important role in proton hopping according to the Grotthuss mechanism.

In connection with acids, hydrogen ions typically refer to hydrons.

Anion (negatively charged)

Hydrogen anions are formed when additional electrons are acquired:

• Hydride: general name referring to the negative ion of any hydrogen isotope
• Deuteride: ²H^-, D^-
• Tritide: ³H^-, T^-

In organic chemistry, a hydrogen atom in a molecule is often referred to simply as a proton. The hydrogen anion has played an important role in quantum physics.

See also

• Acid
• pH