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Concept
Hydron
Résumé
In chemistry, the hydron, informally called proton, is the cationic form of atomic hydrogen, represented with the symbol H+. The general term "hydron", endorsed by the IUPAC, encompasses cations of hydrogen regardless of their isotopic composition: thus it refers collectively to protons (1H+) for the protium isotope, deuterons (2H+ or D+) for the deuterium isotope, and tritons (3H+ or T+) for the tritium isotope.
Unlike most other ions, the hydron consists only of a bare atomic nucleus. The negatively charged counterpart of the hydron is the hydride anion, H−.
Other things being equal, compounds that readily donate hydrons (Brønsted acids, see below) are generally polar, hydrophilic solutes and are often soluble in solvents with high relative static permittivity (dielectric constants). Examples include organic acids like acetic acid (CH3COOH) or methanesulfonic acid (CH3SO3H). However, large nonpolar portions of the molecule may attenuate these properties. Thus, as a result of its alkyl chain, octanoic acid (C7H15COOH) is considerably less hydrophilic compared to acetic acid.
The unsolvated hydron (a completely free or "naked" hydrogen atomic nucleus) does not exist in the condensed (liquid or solid) phase. Although superacids are sometimes said to owe their extraordinary hydron-donating power to the presence of "free hydrons", such a statement is highly misleading: even for a source of "free hydrons" like H2F+, one of the superacidic cations present in the superacid fluoroantimonic acid (HF:SbF5), detachment of a free H+ still comes at an enormous energetic penalty on the order of several hundred kcal/mol. This effectively rules out the possibility of the free hydron being present in solution, even as a fleeting intermediate. For this reason, in liquid strong acids, hydrons are believed to diffuse by sequential transfer from one molecule to the next along a network of hydrogen bonds through what is known as the Grotthuss mechanism.
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