Core Question: Why does $H_{3}PO_{2}$ behave as a monobasic acid even though it has 3 hydrogen atoms?

Simple Explanation: Think of phosphorus as a "gatekeeper" for hydrogen atoms. A hydrogen atom can only leave a molecule as $H^{+}$ (a proton) if it is sitting on an oxygen atom. This is because oxygen is electronegative — it pulls electrons away from the H, making the $H^{+}$ easy to "pull off" by water.

In $H_{3}PO_{2}$, draw the structure: phosphorus is at the centre. One H is attached through oxygen (P-O-H). The other two H atoms are attached directly to phosphorus (P-H). Phosphorus is NOT electronegative enough to weaken the P-H bond sufficiently to release $H^{+}$. So those two P-H hydrogens are "locked in" and will never leave as protons.

Analogy: Imagine three people at a door. One person has a badge (P-OH hydrogen) and can leave whenever they want. The other two people (P-H hydrogens) are glued to the wall — they simply cannot leave.

Visual Rule:

• P-O-H bond → H can ionize → contributes to basicity
• P-H bond → H cannot ionize → does NOT contribute to basicity
• Count ONLY P-OH bonds for basicity — ignore all other H atoms

Test yourself: $H_{3}PO_{4}$ has structure (HO)_{3}P=O → 3 P-OH bonds → tribasic. ✓