Oxonium Ion: H₃O⁺

According to IUPAC nomenclature of organic chemistry, the hydronium ion (H₃O⁺) 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.

In organic chemistry when an alcohol (which contains the hydroxyl functional group), gets protonated by acid (accepts the H⁺ ion), it becomes an alkyloxonium ion.  For example, when ethanol (IUPAC name), which is also commonly named ethyl alcohol (CH₃CH₂-OH), is protonated, it would be called the ethyloxonium ion (CH₃CH₂-OH₂⁺).

As a result of the above definitions, an oxonium ion is any ion with a trivalent oxygen cation. For example, a protonated hydroxyl group is an alkyloxonium ion (as described for the ethyl alcohol above), but not a hydronium.

For more information on the oxonium ion, view: http://en.wikipedia.org/wiki/Hydronium

Other IUPAC names related to oxonium ion: onium.htm