Hydrogen ions

Hydronium

Names
IUPAC name oxonium
Other names hydronium ion
Identifiers
CAS Number	13968-08-6 Y
3D model (Jmol)	Interactive image
ChEBI	CHEBI:29412 N
ChemSpider	109935 N
PubChem	123332
InChI InChI=1S/H2O/h1H2/p+1 Key: XLYOFNOQVPJJNP-UHFFFAOYSA-O
SMILES [OH3+]
Properties
Chemical formula	H3O+
Molar mass	19.02 g/mol
Acidity (pKa)	−1.7
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N  (what is YN ?)
Infobox references

In chemistry, hydronium is the common name for the aqueous cation H
3O+
, the type of oxonium ion produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H⁺) to the surrounding water molecules (H₂O).

It is the amount of hydronium ions relative to hydroxide ions that determines a solution's pH. The molecules in pure water auto-dissociate (i.e.: react with each other) into hydronium and hydroxide ions in the following equilibrium:

In pure water, there is an equal number of hydroxide and hydronium ions, so it is a neutral solution. At 25 °C, water has a pH of 7 (this varies when the temperature changes: see self-ionization of water). A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution.

According to IUPAC nomenclature of organic chemistry, the hydronium ion should be referred to as oxonium.Hydroxonium may also be used unambiguously to identify it. A draft IUPAC proposal also recommends the use of oxonium and oxidanium in organic and inorganic chemistry contexts, respectively.

An oxonium ion is any ion with a trivalent oxygen cation. For example, a protonated hydroxyl group is an oxonium ion, but not a hydronium ion.

Since O+
and N have the same number of electrons, H
3O+
is isoelectronic with ammonia. As shown in the images above, H
3O+
has a trigonal pyramidal molecular geometry with the oxygen atom at its apex. The H–O–H bond angle is approximately 113°, and the center of mass is very close to the oxygen atom. Because the base of the pyramid is made up of three identical hydrogen atoms, the H
3O+
molecule's symmetric top configuration is such that it belongs to the C_3vpoint group. Because of this symmetry and the fact that it has a dipole moment, the rotational selection rules are ΔJ = ±1 and ΔK = 0. The transition dipole lies along the c-axis and, because the negative charge is localized near the oxygen atom, the dipole moment points to the apex, perpendicular to the base plane.