Which of the following statements correctly describes the resonance structures of the sulfur trioxide (SO_3) molecule?

• A. SO_3 has three equivalent resonance structures, each with one S=O double bond and two S–O single bonds, with the double bond rotating among the three oxygens.
• B. SO_3 has only one resonance structure, with all S–O bonds being single bonds.
• C. SO_3 has three resonance structures, each with all S–O bonds as double bonds.
• D. SO_3 has two resonance structures, each with two S=O double bonds and one S–O single bond.

Answer: (A)

Verified step by step guidance

1. Recall that resonance structures are different Lewis structures for the same molecule that differ only in the placement of electrons, not the arrangement of atoms.
2. Draw the Lewis structure of SO_3, noting that sulfur is the central atom bonded to three oxygen atoms.
3. Determine the total number of valence electrons: sulfur has 6, each oxygen has 6, so total valence electrons = 6 + 3×6 = 24.
4. Distribute electrons to satisfy the octet rule for oxygen atoms and minimize formal charges, which leads to one double bond between sulfur and one oxygen, and single bonds with the other two oxygens.
5. Recognize that the double bond can be placed with any of the three oxygen atoms, creating three equivalent resonance structures where the double bond 'rotates' among the oxygens.