Draw all possible resonance structures for SO3, SO32-, and SO2

Draw all possible resonance structures for SO3, SO32-, and SO2. Use the resonance structures to solve the problems below.

(a) Arrange these species in order of increasing S-O bond length (shortest bond first).

SO3 SO32- SO2

SO2 SO32- SO3

SO2 SO32- SO3

(b) Match each species with the number of covalent bonds predicted by Lewis structures to exist between an S atom and an O atom bonded to this S atom. (Hint: Average the number of bonds between S and an attached oxygen atom in a particular position using all of your resonance structures for the species that you are working on.)

SO2: —Select— 1.00 1.33 1.50 2.00 3.00 bonds between S and O.

SO3: —Select— 1.00 1.33 1.50 2.00 3.00 bonds between S and O.

SO32-: —Select— 1.00 1.33 1.50 2.00 3.00 bonds between S and O.

(c) Match each species with the correct formal charge on the central S atom.

SO2: —Select— -1.00 -0.67 -0.50 0.00 +1.00 +2.00 charge on S.

SO3: —Select— -1.00 -0.67 -0.50 0.00 +1.00 +2.00 charge on S.

SO32-: —Select— -1.00 -0.67 -0.50 0.00 +1.00 +2.00 charge on S.

(d) Match each species with the average formal charge on an outside oxygen atom predicted by Lewis structures.

SO2: —Select— -1.00 -0.67 -0.50 0.00 +1.00 +2.00 average charge on outside O atom.

SO3: —Select— -1.00 -0.67 -0.50 0.00 +1.00 +2.00 average charge on outside O atom.

SO32-: —Select— -1.00 -0.67 -0.50 0.00 +1.00 +2.00 average charge on outside O atom.

The correct answer and explanation is :

To answer these questions, let’s first analyze the structures of SO₃, SO₃²⁻, and SO₂. We will look at their resonance structures, bond lengths, formal charges, and bond orders.

Resonance Structures:

1. SO₃ (Sulfur Trioxide): SO₃ has three oxygen atoms bonded to a central sulfur atom. The resonance structures involve double bonds between sulfur and oxygen atoms. Since the molecule is symmetric, each oxygen forms a double bond with sulfur in the resonance forms. There are three equivalent resonance structures.
2. SO₃²⁻ (Sulfite Ion): The sulfite ion has three oxygen atoms, but one oxygen carries a negative charge, and sulfur typically has a formal charge of +1. The resonance structures involve one of the oxygen atoms carrying a -1 charge, and the bonds between sulfur and oxygen alternate between single and double bonds in the resonance forms.
3. SO₂ (Sulfur Dioxide): SO₂ has two oxygen atoms bonded to sulfur, and the molecule is also resonance-stabilized. In the resonance structures, one oxygen forms a double bond, while the other forms a single bond, and the formal charges alternate between the two oxygens.

Answering the Questions:

(a) Order of increasing S-O bond length (shortest bond first):

• SO₃ will have the shortest S-O bond length. Since all the bonds in SO₃ are double bonds in the resonance structure, these bonds are shorter than single bonds.
• SO₂ will have intermediate bond lengths because the bond between sulfur and one oxygen is a double bond, and the other is a single bond.
• SO₃²⁻ will have the longest bond lengths. The resonance structure of SO₃²⁻ involves single and double bonds, but there is a -1 charge on one of the oxygens, making these bonds weaker and longer.

Answer: SO₃ < SO₂ < SO₃²⁻

(b) Number of covalent bonds between S and O in each species:

• SO₂: Average bond order between sulfur and oxygen is 1.5. In the resonance structures, one sulfur-oxygen bond is a double bond and the other is a single bond, averaging to 1.5 bonds.
• SO₃: Average bond order between sulfur and oxygen is 2. In the resonance structures, sulfur forms a double bond with each oxygen atom.
• SO₃²⁻: Average bond order between sulfur and oxygen is 1.33. The sulfite ion has single and double bonds between sulfur and oxygen, with the resonance structures spreading the electron density over all bonds.

Answer:

• SO₂: 1.5 bonds
• SO₃: 2.0 bonds
• SO₃²⁻: 1.33 bonds

(c) Formal charge on the central sulfur atom:

• SO₂: The formal charge on the sulfur atom is 0. The sulfur atom in SO₂ is typically bonded to oxygen with one double bond and one single bond in the resonance structures, which balances the formal charge.
• SO₃: The formal charge on the sulfur atom is 0. In SO₃, sulfur forms three double bonds, so no formal charge is on sulfur.
• SO₃²⁻: The formal charge on the sulfur atom is +1. The sulfite ion has a formal +1 charge on sulfur because of the distribution of the charges on the oxygen atoms.

Answer:

• SO₂: 0.00 charge on sulfur
• SO₃: 0.00 charge on sulfur
• SO₃²⁻: +1.00 charge on sulfur

(d) Average formal charge on an outside oxygen atom:

• SO₂: The average formal charge on an oxygen atom is 0. In the resonance structure, the formal charge is distributed equally between the oxygen atoms, and each oxygen has no formal charge on average.
• SO₃: The average formal charge on an oxygen atom is 0. In the resonance structures, the oxygen atoms share equal bonding and charge distribution, leading to no formal charge on the oxygens.
• SO₃²⁻: The average formal charge on an oxygen atom is -0.67. In the resonance structures, one oxygen carries a -1 charge, and the others carry no charge.

Answer:

• SO₂: 0.00 average charge on oxygen
• SO₃: 0.00 average charge on oxygen
• SO₃²⁻: -0.67 average charge on oxygen

Explanation:

In summary:

• SO₃ has the shortest S-O bonds because all bonds are double bonds in its resonance structures.
• SO₂ has an intermediate bond length, with an average bond order of 1.5 due to alternating single and double bonds.
• SO₃²⁻ has the longest S-O bonds due to partial single bonds and the presence of a negative charge.

The formal charges on sulfur and oxygen follow the same logic. In SO₃ and SO₂, the central sulfur atom has no formal charge, while in SO₃²⁻, sulfur has a +1 charge, and the oxygen atoms carry a -1 charge in one of the resonance structures.