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Does anyone know why the hydrogen atoms bonded with phosphorus in $\ce{H3PO2}$ are not ionizable?

I tried looking in high school level books, but they only describe the process, not the real reason. I would appreciate if someone has the answer.

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    $\begingroup$ See, the P-H bond is not all that polar. $\endgroup$ – Ivan Neretin Nov 23 '19 at 23:08
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    $\begingroup$ Even nitrogen attached hydrogen atom is very difficult to ionize, and phosphorus has almost the same electronegativity as hydrogen. Nonpolar bonds do not get ionized in aqueous solutions. $\endgroup$ – Poutnik Nov 24 '19 at 10:18
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To begin with, removing another proton from anionic species is not very favorable. For example, the monohydrogen phosphate dianion is not very acidic $(\mathrm{p}K_\mathrm{a}\sim 12)$ despite it having an $\ce{OH}$ group.

In the case of $\ce{H3PO2},$ the second and third protons are bonded directly to $\ce{P},$ which is much less electronegative than $\ce{O},$ and consequently these P-H bonds are much less polarized than an $\ce{O-H}$ bond. So these protons are unable to ionize appreciably in aqueous solution.

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  • $\begingroup$ Good point, I revised my answer accordingly. $\endgroup$ – iad22agp Nov 24 '19 at 12:55

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