# 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.

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