Hydron (chemistry)
Encyclopedia
In chemistry, a hydron is the general name for a cationic form of atomic hydrogen
: 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 (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 , 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 solution
s.
The negatively-charged counterpart of the hydron is the hydride
anion, .
Other isotopes of hydrogen
are too unstable to be relevant in chemistry.
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 anion
s 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 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.
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
: most commonly a "proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
". 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
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
.
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
Hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
ion in water-based acids, and H2SbF6+, the unstable cation of fluoroantimonic acid
Fluoroantimonic acid
Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination forms the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H2....
, the strongest superacid
Superacid
According to the classical definition superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function of −12. According to the modern definition, superacid is a medium, in which the chemical potential of the proton is higher than in pure...
. For this reason, in such liquids including liquid acids, hydrons diffuse by contact from one complex cation to another, via the Grotthuss mechanism
Grotthuss mechanism
The Grotthuss mechanism is the mechanism by which an 'excess' proton or protonic defect diffuses through the hydrogen bond network of water molecules or other hydrogen-bonded liquids through the formation or cleavage of covalent bonds....
.
The hydrated form of the hydrogen cation, the hydronium
Hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
(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 , 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 solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
s.
The negatively-charged counterpart of the hydron is the hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
anion, .
Isotopes of hydron
- ProtonProtonThe proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
, 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 deuteriumDeuteriumDeuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
, 2H or D. - Triton, having the symbol 3H+ or T+, is the +1 ion of tritiumTritiumTritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
, 3H or T.
Other isotopes of hydrogen
Isotopes of hydrogen
Hydrogen has three naturally occurring isotopes, sometimes denoted 1H, 2H, and 3H. Other, highly unstable nuclei have been synthesized in the laboratory but not observed in nature. The most stable radioisotope is tritium, with a half-life of 12.32 years...
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 isotopeIsotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
mixtures. The name "proton" refers to isotopically pure
Isotope separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...
1H+.
On the other hand, referring to the hydron as simply hydrogen ion is not recommended because hydrogen anion
Hydrogen anion
The hydrogen anion is a negative hydrogen ion, H−. It is an important constituent of the atmosphere of stars, such as the Sun, where it is the dominant absorber of photons with energies in the range 0.75-4.0 eV, ranging from the infrared into the visible spectrum...
s 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
Kinetic isotope effect
The kinetic isotope effect is the ratio of reaction rates of two different isotopically labeled molecules in a chemical reaction. It is also called "isotope fractionation," although this term is somewhat broader in meaning...
or hydrogen isotopic labeling
Isotopic labeling
Isotopic labeling is a technique for tracking the passage of a sample of substance through a system. The substance is 'labeled' by including unusual isotopes in its chemical composition...
). Otherwise, referring to hydrons as protons is still considered acceptable, for example in such terms as protonation
Protonation
In chemistry, protonation is the addition of a proton to an atom, molecule, or ion. Some classic examples include*the protonation of water by sulfuric acid:*the protonation of isobutene in the formation of a carbocation:2C=CH2 + HBF4 → 3C+ + BF4−*the protonation of ammonia in the...
, deprotonation
Deprotonation
Deprotonation is the removal of a proton from a molecule, forming the conjugate base.The relative ability of a molecule to give up a proton is measured by its pKa value. A low pKa value indicates that the compound is acidic and will easily give up its proton to a base...
, proton pump
Proton pump
A proton pump is an integral membrane protein that is capable of moving protons across a cell membrane, mitochondrion, or other organelle. Mechanisms are based on conformational changes of the protein structure or on the Q cycle.-Function:...
or proton channel
Voltage-gated proton channel
Voltage-gated proton channels are ion channels that have the unique property of opening with depolarization, but in a strongly pH-sensitive manner. The result is that these channels open only when the electrochemical gradient is outward, such that their opening will only allow protons to leave cells...
. 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.
However, although 99.9 % of natural hydrogen nuclei are protons, small amounts are deuterons and rare tritons.
See also
- DeprotonationDeprotonationDeprotonation is the removal of a proton from a molecule, forming the conjugate base.The relative ability of a molecule to give up a proton is measured by its pKa value. A low pKa value indicates that the compound is acidic and will easily give up its proton to a base...
- SuperacidSuperacidAccording to the classical definition superacid is an acid with an acidity greater than that of 100% pure sulfuric acid, which has a Hammett acidity function of −12. According to the modern definition, superacid is a medium, in which the chemical potential of the proton is higher than in pure...
- Dihydrogen cation
- Trihydrogen cation
- Hydrogen ion clusterHydrogen ion clusterA 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 clusters with n up to 120.-Experiments:These can be formed...