Question

H₃PO₃ 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 H₃PO₃ ? If not, give reasons for the same.

Answer 1

Answer:

No, the two given structures of H₃PO₃ cannot be taken as canonical (resonance) forms of the same resonance hybrid.

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Reason:

Resonance structures must differ only in the distribution of electrons and not in the positions of atoms.

In the given two structures of H₃PO₃:

• The positions of hydrogen atoms are different. 
• In one structure, hydrogen is directly bonded to phosphorus (P–H bond). 
• In the other structure, hydrogen is bonded to oxygen (O–H bond).

Since the bonding arrangement of atoms changes, these structures are not related by mere shifting of electrons.

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Correct explanation of H₃PO₃ structure:

In phosphorous acid (H₃PO₃), the correct structure contains:

• One P=O bond 
• Two P–OH bonds 
• One P–H bond

Only the structure with a P–H bond represents the correct Lewis structure of H₃PO₃.

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Conclusion:

The two given structures of H₃PO₃ are not resonance structures because they differ in the positions of atoms. Hence, they cannot be taken as canonical forms of the same resonance hybrid.