An oxonium ion is a positive oxygen cation which has three bonds. For example, protonating a carbonyl gives an oxonium ion, with two bonds to a carbon atom and one bond to a hydrogen atom.* The term oxonium is also used specifically to mean the hydroxonium (common name: hydronium) ion, which is the above type of oxonium, with three hydrogens (H3O+).
Oxonium (often abbreviated to Oxon) is sometimes used in university circles as a Latin name for Oxford (University) in England.
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