In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, the hydron, informally called proton,
is the
cationic form of atomic
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
, represented with the symbol . The general term "hydron", endorsed by the
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
, 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
In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...
anion, .
Properties
Solute properties
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
The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insul ...
(dielectric constants). Examples include organic acids like
acetic acid (CH
3COOH) or
methanesulfonic acid (CH
3SO
3H). However, large nonpolar portions of the molecule may attenuate these properties. Thus, as a result of its alkyl chain,
octanoic acid
Caprylic acid (), also known under the systematic name octanoic acid or C8 Acid, is a saturated fatty acid, medium-chain fatty acid (MCFA). It has the structural formula , and is a colorless oily liquid that is minimally soluble in water with a s ...
(C
7H
15COOH) 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 , one of the superacidic cations present in the superacid
fluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride, containing various cations and anions (the simplest being and ). This substance is a superacid that can be over a billion times stronger than 100% pure sulfuric ...
(HF:SbF
5), detachment of a free 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
The Grotthuss mechanism (also known as proton jumping) is the process by which an 'excess' proton or proton defect diffuses through the hydrogen bond network of water molecules or other hydrogen-bonded liquids through the formation and concomitant ...
.
Acidity
The hydron ion can incorporate an electron pair from a Lewis base into the molecule by adduction:
: + :L →
Because of this capture of the Lewis base (L), the hydron ion has Lewis acidic character. In terms of
Hard/Soft Acid Base (HSAB) theory, the bare hydron is an infinitely hard Lewis acid.
The hydron plays a central role in
Brønsted–Lowry acid–base theory
The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory which was proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. The fundamental concept of this theory ...
: a species that behaves as a hydron donor in a reaction is known as the Brønsted acid, while the species accepting the hydron is known as the Brønsted base. In the generic acid–base reaction shown below, HA is the acid, while B (shown with a lone pair) is the base:
: + :B → + :A
–
The hydrated form of the hydrogen cation, the
hydronium
In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid ...
(hydroxonium) ion
(aq), is a key object of
Arrhenius' definition of acid. Other hydrated forms, the
Zundel cation , which is formed from a proton and two water molecules, and the
Eigen cation , which is formed from a hydronium ion and three water molecules, are theorized to play an important role in the diffusion of protons though an
aqueous solution according to the Grotthuss mechanism. Although the ion
(aq) is often shown in introductory textbooks to emphasize that the hydron is never present as an unsolvated species in aqueous solution, it is somewhat misleading, as it oversimplifies infamously complex speciation of the solvated proton in water; the notation
(aq) is often preferred, since it conveys aqueous solvation while remaining noncommittal with respect to the number of water molecules involved.
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
Triton commonly refers to:
* Triton (mythology), a Greek god
* Triton (moon), a satellite of Neptune
Triton may also refer to:
Biology
* Triton cockatoo, a parrot
* Triton (gastropod), a group of sea snails
* ''Triton'', a synonym of ''Triturus'' ...
, 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
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
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
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
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.
The term "hydron" was defined by
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
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
In physical organic chemistry, a kinetic isotope effect (KIE) is the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes. Formally, it is the ratio of rate constants for ...
or hydrogen
isotopic labeling). Otherwise, referring to hydrons as protons is still considered acceptable, for example in such terms as
protonation
In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...
,
deprotonation
Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju. ...
,
proton pump, or
proton channel
A proton pump is an integral membrane protein pump that builds up a proton gradient across a biological membrane. Proton pumps catalyze the following reaction:
: n one side of a biological membrane/sub> + energy n the other side of the membr ...
. The transfer of in an acid-base reaction is usually referred to as ''proton'' transfer. Acid and bases are referred to as ''proton'' donors and acceptors correspondingly.
99.9844% of natural hydrons (hydrogen nuclei) are protons, and the remainder (about 156 per million in sea water) are deuterons (see
deuterium), except for some very rare natural tritons (see
tritium).
See also
*
Deprotonation
Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju. ...
*
Dihydrogen cation
The dihydrogen cation or hydrogen molecular ion is a cation (positive ion) with formula . It consists of two hydrogen nuclei ( protons) sharing a single electron. It is the simplest molecular ion.
The ion can be formed from the ionization of ...
*
Hydrogen ion cluster A hydrogen molecular ion cluster or hydrogen cluster ion is a positively charged cluster of hydrogen molecules. The hydrogen molecular ion () and trihydrogen ion () are well defined molecular species. However hydrogen also forms singly charged clust ...
*
Solvated electron
*
Superacid
*
Trihydrogen cation
References
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Cations
Hydrogen
Proton