Draw the resonance structure that has the lowest formal charge on each atom for SO3-2

Draw the resonance structure that has the lowest formal charge on each atom for SO3-2. What is the formal charge on sulfur?

The correct answer and explanation is:

Let’s analyze the resonance structures of the sulfite ion, SO₃²⁻, and find the one with the lowest formal charges on each atom, especially on sulfur.

Step 1: Determine the total number of valence electrons

• Sulfur (S) has 6 valence electrons.
• Each oxygen (O) has 6 valence electrons, and there are 3 oxygens → 3 × 6 = 18 electrons.
• The ion has a charge of -2, so add 2 more electrons.

Total valence electrons: 6 + 18 + 2 = 26 electrons.

Step 2: Draw a basic Lewis structure

• Place sulfur in the center connected to three oxygen atoms.
• Connect sulfur to each oxygen with a single bond initially.
• Distribute remaining electrons to complete octets on oxygens, then sulfur.

Step 3: Construct resonance structures

The sulfite ion has three resonance structures where one of the oxygens forms a double bond with sulfur, and the other two oxygens have single bonds.

In these resonance forms:

• One oxygen has a double bond with sulfur (S=O).
• The other two oxygens have single bonds and each carry a negative charge.
• Sulfur has a lone pair.

Step 4: Calculate formal charges in this resonance structure

Recall:

Formal charge = (Valence electrons) – (Nonbonding electrons) – (Bonding electrons / 2)

• For sulfur (S):
  • Valence electrons = 6
  • Nonbonding electrons = 2 (one lone pair)
  • Bonding electrons = 8 (two single bonds + one double bond: 2+2+4 = 8)
  Formal charge = 6 – 2 – (8/2) = 6 – 2 – 4 = 0
• For double-bonded oxygen (O):
  • Valence electrons = 6
  • Nonbonding electrons = 4 (two lone pairs)
  • Bonding electrons = 4 (double bond)
  Formal charge = 6 – 4 – (4/2) = 6 – 4 – 2 = 0
• For each single-bonded oxygen (O):
  • Valence electrons = 6
  • Nonbonding electrons = 6 (three lone pairs)
  • Bonding electrons = 2 (single bond)
  Formal charge = 6 – 6 – (2/2) = 6 – 6 – 1 = -1

Step 5: Conclusion

• The resonance structure with one S=O double bond and two S–O single bonds bearing negative charges on the oxygens has the lowest formal charge on sulfur: 0.
• The sulfur atom in this resonance structure has a formal charge of 0.
• The negative charges are localized on the two single-bonded oxygens, which is consistent with oxygen’s higher electronegativity.

Summary

The sulfite ion (SO₃²⁻) has three major resonance structures with one double bond between sulfur and an oxygen and two single bonds with oxygens carrying negative charges. The sulfur atom in these resonance structures has a formal charge of zero, which is the lowest formal charge sulfur can have in this ion.