Worked example 1
Compare the two common Lewis structures of the sulfate ion SO₄ 2-.
Try it first: Count the 32 valence electrons and try the all single bond structure first.
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Molecular StructureFormal ChargeContent level: Challenge 22 min
Expanded Octets and Formal Charge Trade-offs
What you'll be able to do: Decide when expanding an octet is justified and weigh the formal charge argument against the octet argument.
Best after: Choosing the Best Lewis Structure with Formal Charge, Exceptions to the Octet Rule
Introduction
Sulfate can be drawn with four single bonds or with two double bonds. Both appear in textbooks, and knowing why is the point of this lesson.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Identify central atoms that may expand their octet
- Compare formal charges for octet and expanded octet structures
- Explain the trade-off between minimising formal charge and keeping an octet
- Write a justified answer that names the criterion used
Lesson
The sulfate example
With four S-O single bonds, sulfur has an octet but carries a formal charge of +2 while each oxygen carries -1. Converting two of those bonds to double bonds gives sulfur twelve electrons and drops its formal charge to zero, leaving only two oxygens at -1.
Both drawings account for all 32 valence electrons. They differ only in how the electrons are arranged.
The trade-off
Rule one of formal charge ranking prefers the expanded structure. The octet rule prefers the strict one. Computational studies of sulfate show the S-O bonding is largely ionic with little d orbital participation, supporting the strict octet form, yet the expanded form remains standard in many textbooks because it reproduces the short observed S-O bond lengths.
Sulfur dioxide
SO₂ with one single and one double bond gives sulfur +1 and the single bonded oxygen -1. Drawing both bonds double gives every atom a formal charge of zero at the cost of ten electrons on sulfur. The same argument applies, and the same two answers appear in different sources.
How to answer safely
State the structure, give the formal charges, and name the criterion. For example: minimising formal charge favours the expanded octet with two S=O bonds, while strict octet reasoning favours four single bonds with sulfur at +2. Naming the criterion earns credit either way.
This flexibility applies only to period 3 and heavier centres. A period 2 atom never gets the choice.
Know what an expanded-octet drawing represents
A period-3 central atom can be drawn with more than eight electrons in some useful Lewis structures, but the drawing is a bookkeeping model of bonding, not evidence that literal d-orbital hybridization must occur. Second-period atoms such as N and O cannot exceed an octet in ordinary Lewis structures. Count electrons and check formal charges before choosing among valid drawings.
Period and total valence electrons constrain what Lewis model is possible.
Key ideas
Rule
Who may expand
Only central atoms from period 3 downward, such as P, S, Cl, Br, I and Xe.
Key concept
Sulfate two ways
Four single bonds gives S at +2; two double bonds gives S at 0 with twelve electrons.
Rule
Name the criterion
State whether you are minimising formal charge or preserving the octet.
Definition
Charge separation
Having several nonzero formal charges within one species, which usually costs energy.
Equation
Formal charge
FC = V - N - B/2
Worked examples
Worked example 2
Draw SO₂ so that all formal charges are zero, and state the cost.
Try it first: Count the 18 valence electrons.
0 of 3 steps revealed.
Worked example 3
Why can PF₅ be drawn with five P-F bonds while NF₅ is not a comparable ordinary molecule?
Try it first: Compare the central atoms' periods and their valence-shell electron limits.
0 of 3 steps revealed.
Common mistakes
Expanding the octet of oxygen or nitrogen to zero the formal charges.
Why it's wrong: Period 2 atoms cannot exceed eight electrons under any argument.
Check instead: Only expand a central atom in period 3 or below.
Claiming one sulfate structure is simply wrong.
Why it's wrong: Both are defensible depending on the criterion applied.
Check instead: Name the criterion in your answer.
Adding double bonds without recounting the electron total.
Why it's wrong: The valence budget is fixed, so a new bond must come from an existing lone pair.
Check instead: Recount bonding and non-bonding electrons after each change.
After the lesson · Question bank
Independent practice
Work through new problems on this topic on your own. The lesson's quick checks and worked examples stay above if you need to review a step.
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What you should now know
For period 3 and heavier central atoms, expanding the octet can reduce formal charges dramatically, as in sulfate and sulfur dioxide. Modern calculations favour the strict octet form with charge separation, while many courses still teach the expanded form, so the defensible answer states the criterion being used.
- Expanded octets can zero out large formal charges on period 3 centres
- Sulfate and SO₂ each have two defensible structures
- Modern calculations support strict octets with charge separation
- Textbooks often show the expanded form for shorter bond lengths
- Always name the criterion used when justifying a choice
Sources and further reading
- Chemistry 2e, Section 7.4: Formal Charges and ResonanceOpenStax · Rice University · 7.4View source
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