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An 'oxoacid' is an acid which contains oxygen. More specifically, it is an acid which:
#contains oxygen;
#contains at least one other element;
#has at least one hydrogen atom bound to oxygen; and
#forms an ion by the loss of one or more protons.
The name 'oxyacid' is sometimes used, although this is not recommended.
Generally, oxoacids are simply polyatomic ions with a hydrogen cation.
Under Lavoisier's original theory, all acids contained oxygen, which was named from the Greek οξυς (''oxys'') (acid, sharp) and γεινομαι (''geinomai'') (engender). It was later discovered that some acids, notably hydrochloric acid, did not contain oxygen and so a distinction was made for those that did.
Common oxoacids include:

★ Halogen oxoacids: Hypochlorous acid ; Chlorous acid; Chloric acid; Perchloric acid; Perbromic acid; Metaperiodic acid

★ Sulfuric acid

★ Nitric acid

★ Phosphoric acid
Common acids which are not oxoacids include:

★ Hydrochloric acid

★ Hydrofluoric acid

★ Hydrobromic acid

★ Hydroiodic acid
Although carboxylic acids fulfill the criteria above, they are not generally considered as oxoacids.
All oxoacids have the acidic hydrogen bound to an oxygen atom, so bond strength (length) is not a factor as it is with binary nonmetal hydrides. Rather, the electronegativity of the central atom (E) and the number of O atoms determine oxoacid acidity. With the same number of oxygens around E, acid strength increases with the electronegativity of E.

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Note

# This final criterion has the effect of excluding boric acid from the strict definition, as it forms its anion by addition of hydroxide rather than loss of a proton: B(OH)₃ + H₂O  [B(OH)₄]⁻ + H⁺. However, boric acid is usually considered to be an oxoacid nonetheless.

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★ IUPAC definition of "oxoacid" (from the "''Gold Book''")