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Understanding the Context

What Is the Sulphate Ion?

The sulphate ion is formed when sulfur (S) reacts with oxygen (O), typically losing two electrons to achieve a +6 oxidation state in a sulfonate group. The molecular formula is SO₄²⁻, indicating one sulfur atom covalently bonded to four oxygen atoms with a net negative charge. This polyatomic ion appears in many compounds, including gypsum (CaSO₄), detergents, and wastewater treatment agents.

Step-by-Step Construction of the Lewis Structure

Key Insights

To determine the Lewis structure, follow these systematic steps:

1. Count Total Valence Electrons

• Sulfur (S) is in Group 16: 6 valence electrons
  
• Each oxygen (O) is in Group 16: 6 valence electrons × 4 = 24
  
• Add 2 electrons for the 2 negative charges
  
• Total = 6 + 24 + 2 = 32 valence electrons

2. Determine the Central Atom

• Sulfur is less electronegative than oxygen and lies in the center of the ion, forming a central sulfur surrounded by oxygen atoms.

3. Form Single Bonds Between S and Each Oxygen

• Place a single bond (sharing 2 electrons) between sulfur and each of the four oxygen atoms:
  4 bonds × 2 electrons = 8 bonding electrons
  
• Remaining electrons: 32 – 8 = 24 electrons → 12 lone pairs

4. Distribute Lone Pairs to Account for Octet Rule

• Each oxygen needs 6 more electrons (total 8 for a complete octet):
  4 oxygens × 6 electrons = 24 electrons → all lone pairs accounted for
  
• At this stage, sulfur has 2 bonding pairs and would only have 6 electrons unless expanded.

Final Thoughts

5. Calculate Formal Charges

• Formal charge = Valence electrons – (Lone pair electrons + ½ Bonding electrons)
  
• For sulfur:
  6 – (2 + ½ × 8) = 6 – (2 + 4) = 0
  
• For oxygen:
  Each oxygen has 6 lone pair electrons and 1 shared bond (2 electrons):
  6 – (6 + ½ × 2) = 6 – (6 + 1) = –1
  
• Total formal charge = 0 (S) + 4 × (–1) = –2, matching the ion’s charge.

6. Review Bonding and Expansion Possibility

• Sulfur in period 3 can expand its octet due to available d-orbitals. However, in sulphate, sulfur forms only 4 bonds using sp³ hybridization, resulting in a tetrahedral electron geometry.
  
• All four S–O bonds are equivalent in energy due to resonance (explained below), stabilizing the ion.

7. Include Resonance Structures

The true structure of SO₄²⁻ is a hybrid of multiple resonance forms, even though standard Lewis structures often depict one S–O bond as double bond with formal charges. A more accurate depiction involves delocalization:

• All four S–O bonds have equal character (partial double bond strength).
  
• Electrons are distributed across all S–O bonds via resonance, resulting in bond order averaging of 1.5 per bond.
  
• This delocalization contributes to the ion’s stability, reducing charge separation and delivering equal bond lengths close to ideal tetrahedral angles (~109.5°).