The metaphosphate ion, PO3 – , is the structural analog of the NO3 – ion with respect to the arrangement of the atoms in the ion. After drawing the “best” Lewis structure for the metaphosphate ion based on formal charge considerations, what is the number of resonance structures for this “best” Lewis structure? a. 1 (no resonance) b. 2 c. 3 d. 4 e. 5
The Correct Answer and Explanation is:
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Correct answer: c. 3
To determine the number of resonance structures for the metaphosphate ion (PO₃⁻), we begin by drawing the best Lewis structure using formal charge considerations.
Step 1: Count the total valence electrons
- Phosphorus (P) has 5 valence electrons
- Oxygen (O) has 6 valence electrons × 3 = 18
- The ion has a -1 charge, which adds 1 more electron
Total = 5 + 18 + 1 = 24 valence electrons
Step 2: Draw the skeleton structure
We place phosphorus (the least electronegative atom) in the center with three oxygen atoms surrounding it, forming single bonds initially. This uses up 3 × 2 = 6 electrons. The remaining 18 electrons are placed as lone pairs on the oxygens to complete their octets.
Now phosphorus has only 6 electrons around it. Phosphorus can expand its octet (being in Period 3), so to reduce formal charges and stabilize the structure, we can form a double bond between phosphorus and one of the oxygen atoms.
Step 3: Determine the formal charges
With one double bond and two single bonds:
- The double bonded oxygen has a formal charge of 0
- Each single bonded oxygen has a formal charge of -1
- The phosphorus has a formal charge of +1
Total formal charge = -1, matching the ion’s charge. This structure minimizes formal charges and is considered the “best” Lewis structure.
Step 4: Resonance structures
Because any of the three oxygen atoms can form the double bond with phosphorus, we can draw three equivalent resonance structures by rotating the position of the double bond.
Conclusion
There are 3 resonance structures for the metaphosphate ion, each equivalent in energy and structure. These structures represent the delocalization of electrons in the molecule, and the actual ion is a resonance hybrid of all three.
Correct choice: c. 3
