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.
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$\begingroup$ chemistry.stackexchange.com/questions/44210/structure-of-h3po3 $\endgroup$– MithoronCommented Mar 11, 2016 at 22:12
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1 Answer
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The two isomers you're referring to both exist and are in equilibrium:
When only one or two protons are removed, resonance stabilizes the anions formed, e.g. the $\ce{HPHO3^-}$ anion:
The trianion (shown below) doesn't have the benefit of resonance stabilization. This makes removing the third proton much harder.
answered Mar 11, 2016 at 22:21