Q.$H_3PO_3$ can be represented by structures 1 and 2 shown below. Can these two structures be taken as the canonical forms of the resonance hybrid representing $H_3PO_3$? If not, give reasons for the same.

The key idea is that resonance structures must differ only in the placement of π electrons or lone pairs — the positions of all atoms and sigma bonds must remain identical.

The two given structures, read from the actual valence-electron dot diagrams printed in the book:

• Structure (1): $H–P(=O)(OH)_2$ — one hydrogen is bonded directly to phosphorus (a P–H bond), plus two P–OH groups and one P=O.
• Structure (2): $P(OH)_3$ — all three hydrogens are bonded to oxygen (three P–OH groups); phosphorus carries a lone pair instead, and there is no P–H bond at all.

Step 1: Compare the two structures atom by atom. In (1), the third hydrogen sits on phosphorus. In (2), that same hydrogen sits on the third oxygen instead — everything else about the skeleton is the same.

Step 2: This is a change in atomic connectivity (which atom the hydrogen is bonded to), not just a change in where a π electron or lone pair sits. Resonance forms must share an identical atomic skeleton; only electrons may be redistributed between them. …

What resonance actually requires

Two (or more) Lewis structures qualify as resonance/canonical forms of the same species only if they satisfy one strict rule: every atom occupies exactly the same position in every structure — only the electrons (bonding pairs, lone pairs) are redistributed. If moving from one structure to the other requires relocating even a single atom, the two structures represent different chemical species (isomers or tautomers), not resonance forms of one species.

Reading the two given structures

Structure (1): phosphorus is bonded to one hydrogen directly (a P–H bond), two –O–H groups, and one terminal oxygen — written out, $H–P(=O)(OH)_2$. All three of the molecule's hydrogens are accounted for: one sits directly on phosphorus, and two sit on oxygen.

Structure (2): phosphorus is bonded to three –O–H groups only — $P(OH)_3$ — with a lone pair on phosphorus and no direct P–H bond at all. Here all three hydrogens sit on oxygen.

The key difference

Compare the two atom by atom. The molecular formula ($H_3PO_3$) and the overall bonding framework (one P, three O, three H) are the same in both — but the specific hydrogen that in structure (1) is bonded straight to phosphorus is, in structure (2), instead bonded to the third oxygen. Getting from (1) to (2) means physically moving that hydrogen atom from P to O — this is not something resonance can do. Resonance only ever moves π electrons or lone pairs; it never moves a σ-bonded atom.

Why this means they're not resonance forms

Because the atomic connectivity differs, structures (1) and (2) describe two chemically distinct species — a proton (H⁺) has effectively migrated from phosphorus to oxygen. This makes them tautomers: isomers that interconvert by the movement of a single atom (usually H), not resonance contributors of one hybrid structure. …