Draw all possible resonance structures for S3, [SO3]2-, and SO2. Use the resonance structures to solve the problems below.
(b) Match each species with the number of covalent bonds predicted by the Lewis structures to exist between an S atom and an O atom bonded to this S atom.
Options are 1.00, 1.33, 1.50, 2.00 or 3.00.
**(SO2 and SO3 don’t have 2.00 or 3.00 bonds between S and O.)
**([SO3]2- doesn’t have 1.33 or 2.00 bonds.)
The Correct Answer and Explanation is :
Resonance Structures for the Species:
1. ( S_3 ) (Triatomic Sulfur):
- Sulfur atoms are in a chain (linear or bent) and connected by single and double bonds.
- Resonance structures:
- S=S–S ↔ S–S=S
- Bond order: Each bond alternates between single and double, resulting in an average bond order of 1.5 for S-S bonds.
2. [SO_3]^{2-} (Sulfite Ion):
- Three resonance structures where the negative charge is delocalized over the three oxygens.
- Example structure:
- One S=O double bond and two S–O single bonds with lone pairs on the oxygens.
- Resonance structures: ↔ S=O, S–O, S–O ↔ (negative charge on different oxygens).
- Bond order: The average bond order is ( (1 \text{ double bond + 2 single bonds})/3 = 1.33 ).
3. SO_2 (Sulfur Dioxide):
- Two resonance structures with one S=O double bond and one S–O single bond in each, with lone pairs on the oxygen atoms.
- Resonance structures: ↔ S=O, S–O ↔ (oxygen atoms switch roles for the double bond).
- Bond order: The average bond order is ( (1 \text{ double bond + 1 single bond})/2 = 1.5 ).
Matching Species with Bond Order:
- ( S_3 ):
- Bond order between sulfur atoms = 1.5.
- [SO_3]^{2-}:
- Bond order between sulfur and oxygen atoms = 1.33.
- SO_2:
- Bond order between sulfur and oxygen atoms = 1.5.
Explanation (300 Words):
Resonance structures provide insights into the distribution of bonding electrons across a molecule. They are particularly useful for understanding bond orders, which describe the average number of bonds between two atoms. For ( S_3 ), the bonding alternates between single and double bonds, leading to a resonance hybrid where each S-S bond is an average of 1.5 bonds.
The sulfite ion, ([SO_3]^{2-}), has three equivalent resonance structures. In each structure, sulfur forms one double bond with oxygen while the remaining two oxygens are singly bonded and bear negative charges. The delocalization of the negative charges results in an average bond order of 1.33 for each S-O bond.
Sulfur dioxide (SO₂) has two significant resonance structures. In each, one oxygen forms a double bond with sulfur while the other forms a single bond. The resonance hybrid averages these configurations, giving an S-O bond order of 1.5.
By matching the calculated bond orders with the options given:
- ( S_3 ) matches 1.5 (covalent bonds between S atoms).
- ([SO_3]^{2-}) matches 1.33 (covalent bonds between S and O atoms).
- ( SO_2 ) matches 1.5 (covalent bonds between S and O atoms).
This reasoning avoids assigning incorrect bond orders (like 2.00 or 3.00) that do not align with the resonance and delocalization evident in these molecules.