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+(aq), 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−.

Isotopes of hydron
Proton, having the symbol p or 1H+, is the +1 ion of protium, 1H.
Deuteron, having the symbol 2H+ or D+, is the +1 ion of deuterium, 2H or D.
Triton?, having the symbol 3H+ or T+, is the +1 ion of tritium, 3H or T.
Other isotopes of hydrogen are too unstable to be relevant in chemistry.

History of the term
The term "hydron" is recommended by IUPAC to be used instead of "proton" if no distinction is made between the isotopes proton, deuteron and triton?, all found in naturally occurring undifferentiated isotope mixtures. The name "proton" refers to isotopically pure 1H+. On the other hand, referring to the hydron as simply hydrogen ion is not recommended because hydrogen anions also exist.

Hydron was defined by IUPAC in 1988. Traditionally, the term "proton" was and is used in place of "hydron". The latter term is generally only used in the context where comparisons between the various isotopes of hydrogen is important (as in the kinetic isotope effect or hydrogen isotopic labeling). Otherwise, referring to hydrons as protons is still considered acceptable, for example in such terms as protonation, deprotonation?, proton pump or proton channel. The transfer of H+ in an acid-base reaction is usually referred to as proton transfer. Acid and bases are referred to as proton donors and acceptors correspondingly.

However, although 99.9 % of natural hydrogen nuclei are protons, small amounts are deuterons and rare tritons. Wikipedia, Hydron (external link)

Photos and Graphics of the Hydron Nucleus (external link)

See Also

Inert Gas

Page last modified on Friday 28 of March, 2014 12:53:50 MDT

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