How many double bonds are present in the most stable (best) resonance structure of the phosphate ion, PO_4^{3-}?

• A. 4
• B. 3
• C. 1
• D. 2

Answer: (C)

Verified step by step guidance

1. Step 1: Determine the total number of valence electrons for the phosphate ion, PO_4^{3-}. Phosphorus (P) has 5 valence electrons, each oxygen (O) has 6 valence electrons, and the ion has an extra 3 electrons due to the 3- charge. Calculate the total: \$5 + 4 \times 6 + 3$.
2. Step 2: Draw the skeletal structure of PO_4^{3-} with phosphorus as the central atom bonded to four oxygen atoms. Connect each oxygen to phosphorus with a single bond initially.
3. Step 3: Distribute the remaining valence electrons as lone pairs on the oxygen atoms to satisfy the octet rule for each oxygen. Count how many electrons have been used so far (bonds and lone pairs).
4. Step 4: Check the formal charges on each atom. The goal is to minimize formal charges and have the most stable resonance structure. If oxygens have a formal charge less than zero and phosphorus has a positive formal charge, consider forming double bonds between phosphorus and one or more oxygens to reduce formal charges.
5. Step 5: Identify the resonance structure with the lowest formal charges and the most stable arrangement. In the case of PO_4^{3-}, this structure typically has one P=O double bond and three P–O single bonds, resulting in one double bond in the most stable resonance form.