# Why is phosphorous acid diprotic and not triprotic? [closed]

In H3PO3 only only 2 hydroxide groups exist, instead of 3. If there is 3 hydroxide group, the phosphorous will have 1 lone pair electrons, which will still satisfy the valence electron of Phosphorous.

## closed as unclear what you're asking by hBy2Py, Jon Custer, ron, jerepierre, orthocresol♦Mar 11 '16 at 23:09

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When only one or two protons are removed, resonance stabilizes the anions formed, e.g. the $\ce{HPHO3^-}$ anion: