How many ground-state resonance structures does the sulfur trioxide (SO3) have? Assume all atoms in each structure meet the octet rule.
A Only 1 structure is possible
B 2
C 3
D 4
The correct answer and explanation is :
The correct answer is:
C) 3
Explanation:
Sulfur trioxide (SO₃) is a molecule with a central sulfur atom bonded to three oxygen atoms. To determine how many ground-state resonance structures SO₃ has, we need to understand the bonding and electron distribution consistent with the octet rule.
Step 1: Basic Structure and Octet Rule
- Sulfur (S) has 6 valence electrons.
- Each oxygen (O) has 6 valence electrons.
- Total valence electrons: 6 (S) + 3 × 6 (O) = 24 electrons.
The Lewis structure for SO₃ must distribute these electrons to satisfy the octet rule (8 electrons in the valence shell) around each atom.
Step 2: Resonance Structures of SO₃
- In the simplest Lewis structure, sulfur is bonded to each oxygen by a single bond, and each oxygen has three lone pairs. However, this structure places a positive charge on sulfur and negative charges on the oxygens, which is less stable.
- To satisfy the octet rule and reduce formal charges, sulfur forms double bonds with oxygen atoms. Because sulfur can expand its octet (period 3 element), it can have more than 8 electrons in its valence shell.
- The most stable resonance forms are those with sulfur forming one double bond with oxygen and single bonds with the other two oxygens, but the double bond can be placed with any of the three oxygen atoms.
Step 3: Counting Resonance Structures
- Since the double bond can be placed with oxygen atom 1, or oxygen atom 2, or oxygen atom 3, there are exactly 3 resonance structures.
- Each resonance structure differs by the location of the double bond, while the rest of the atoms maintain single bonds and lone pairs accordingly.
Step 4: Summary of Resonance in SO₃
- The resonance structures are equivalent and contribute equally to the resonance hybrid.
- All structures obey the octet rule for each atom.
- The actual bonding is best described as a resonance hybrid of these 3 structures, with partial double-bond character distributed over all three S–O bonds, making all S–O bonds equivalent.
In conclusion:
Because the double bond between sulfur and oxygen can be placed with any one of the three oxygen atoms, SO₃ has 3 resonance structures that satisfy the octet rule and represent its ground-state resonance forms. Thus, the answer is C) 3.