SO₃ Lewis Structure (Sulfur Trioxide)
SO₃ (sulfur trioxide) has 24 valence electrons and a trigonal planar molecular geometry with 120° bond angles. One common way of drawing its Lewis structure shows sulfur double-bonded to all three oxygen atoms, giving an expanded octet of 12 electrons around sulfur and zero formal charges. If a course requires sulfur to keep a strict octet, SO₃ can instead be drawn as three equivalent charge-separated resonance structures, each with one S=O double bond and two S–O single bonds.
Not sulfite
SO₃ is neutral sulfur trioxide, not the sulfite ion SO₃²⁻. Sulfite has two additional valence electrons, an overall 2− charge, and a different Lewis structure and molecular geometry. Draw SO₃²⁻ in the Lewis Structure Generator →Lewis Structure
Three S=O double bonds. Lone pairs: 2 on each oxygen.
Molecular Geometry
Trigonal planar shape · 120° · AX₃
| Total valence electrons | 24 |
|---|---|
| Central atom | Sulfur (S) |
| Bonds | Three S=O double bonds |
| Lone pairs on S | 0 |
| Lone pairs on O | 2 |
| Electron geometry | Trigonal planar |
| Molecular geometry | Trigonal planar |
| VSEPR | AX₃ |
| Bond angle | 120° |
| Polarity | Nonpolar |
| S hybridization | sp² |
| Resonance | 3 strict-octet resonance forms |
| Formal charges | 0 (expanded form) |
Important
Different textbooks use different Lewis-structure conventions for SO₃. The common expanded-octet drawing has three S=O bonds and zero formal charges. A strict-octet treatment instead uses three equivalent charge-separated resonance contributors. Both point to the same trigonal planar molecular geometry. See the octet-preserving resonance forms →How to Draw the SO₃ Lewis Structure
Draw the SO₃ Lewis structure step by step. Count 24 valence electrons, put sulfur in the center, complete the oxygen octets, then compare the common expanded-octet drawing with the strict-octet resonance contributors.
Step 1Count the valence electrons
Sulfur contributes 6 valence electrons and each oxygen contributes 6. SO₃ is neutral, so the total is 6 + 3(6) = 24 electrons to place. Every valid Lewis drawing of sulfur trioxide uses all 24.
Atom Count Valence e⁻ Total S 1 6 6 O 3 6 18 Total 24 Step 2Place sulfur in the center
Sulfur is less electronegative than oxygen, so it is the central atom. Connect sulfur to all three oxygens with single bonds first. The three S–O bonds use 6 electrons, leaving 18.
Step 3Complete the oxygen octets
Give each oxygen three lone pairs. Those 18 electrons complete all three outer octets. Sulfur is then surrounded by only six bonding electrons, so the structure is not yet the preferred drawing.
Step 4Form multiple bonds under the chosen convention
Move a lone pair from one oxygen into an S–O bond to make S=O. That gives sulfur an octet. Under a strict-octet convention, stop with one S=O and two S–O bonds; the double bond can be placed on any oxygen, giving three equivalent charge-separated contributors. If sulfur is allowed an expanded octet, convert a lone pair on each remaining oxygen into a bond too, giving three S=O double bonds.
Step 5Verify electrons and formal charges
Both conventions still use 24 electrons. The expanded-octet drawing has 12 electrons around sulfur and formal charges of 0 throughout. In each strict-octet contributor sulfur has 8 electrons and formal charge +2; the double-bonded oxygen is 0 and each single-bonded oxygen is −1. The formal charges sum to 0 in either representation.
- 24 valence electrons placed
- Common expanded form: three S=O double bonds and six lone pairs total
- Expanded form formal charges: 0 on sulfur and every oxygen
- Strict-octet contributors: one S=O and two S–O bonds
- Each strict-octet contributor has S +2 and two O atoms at −1
- Formal charges sum to 0 in every representation
SO₃ Valence Electrons
Sulfur contributes 6 valence electrons. The 3 oxygen atoms contribute 3 × 6 = 18 valence electrons. Added together, SO₃ starts with 24 valence electrons to place, and as a neutral molecule there is no charge to add or remove. Of those, 12 fill the bonds and 12 sit as lone pairs (2 on each oxygen).
| sulfur: 6 valence electrons |
| 3 × oxygen: 3 × 6 = 18 |
| Total: 6 + 18 = 24 |
SO₃ Resonance Structures
A strict-octet treatment of SO₃ has three equivalent contributors. In each one, a different oxygen is double-bonded to sulfur while the other two are single-bonded. Sulfur has formal charge +2; each single-bonded oxygen has −1. The drawings do not describe fixed single and double bonds in separate molecules. They are contributors to an electron-delocalized description in which the three S–O positions are equivalent.
Octet-preserving resonance contributors
Each of the 3 equivalent forms keeps eight electrons on sulfur. In every contributor, S has a formal charge of +2. The bond placements and outer-atom formal charges vary across the contributors. The resonance arrows show that the oxygens are equivalent in the hybrid.
Expanded-octet representation
If sulfur may expand its valence shell, one drawing is enough: two equal double bonds, formal charge 0 on every atom, and 12 electrons around S. The two oxygens remain equivalent without drawing a second contributor.
Resonance hybrid
Average bond order from the three strict-octet drawings: (2 + 1 + 1) / 3 = 4/3
This 4/3 value is an average across the three strict-octet drawings. It does not mean a real S–O bond is permanently one fixed type; the three-double-bond drawing uses a different Lewis-structure convention.
Which SO₃ Lewis Structure Is Correct?
Both common drawings are valid Lewis conventions, not competing pictures that make different shape predictions. The expanded-octet form minimizes formal charges, while the strict-octet forms keep sulfur at eight electrons and make the charge separation explicit. Neither contributor means one S–O bond stays permanently different from the others.
| Representation | S octet | Formal charges | What it emphasizes |
|---|---|---|---|
| 3 × S=O | Expanded: 12 e⁻ | 0 on all atoms | Common expanded-octet depiction with no formal-charge separation |
| S=O + 2 × S–O (3 contributors) | 8 e⁻ | S +2; two O −1 | Strict-octet resonance contributors with charge separation |
SO₃ Lewis Structure with Formal Charges
Formula
Formal charge = valence e⁻ − nonbonding e⁻ − ½ × bonding e⁻
Expanded-octet structure
S: 6 − 0 − ½(12) = 0 · O: 6 − 4 − ½(4) = 0 · O: 6 − 4 − ½(4) = 0 · O: 6 − 4 − ½(4) = 0
| Atom | Calculation | Formal charge |
|---|---|---|
| S | 6 − 0 − ½(12) = 0 | 0 |
| O | 6 − 4 − ½(4) = 0 | 0 |
| O | 6 − 4 − ½(4) = 0 | 0 |
| O | 6 − 4 − ½(4) = 0 | 0 |
Octet resonance contributor
One of 3 equivalent contributors is shown; the 2 other structures place the multiple bond(s) and formal charges on different outer atoms. S: 6 − 0 − ½(8) = +2 · O: 6 − 6 − ½(2) = −1 · O: 6 − 6 − ½(2) = −1 · O: 6 − 4 − ½(4) = 0
| Atom | Calculation | Formal charge |
|---|---|---|
| S | 6 − 0 − ½(8) = +2 | +2 |
| O | 6 − 6 − ½(2) = −1 | −1 |
| O | 6 − 6 − ½(2) = −1 | −1 |
| O | 6 − 4 − ½(4) = 0 | 0 |
Formal charge = valence electrons − nonbonding electrons − ½(bonding electrons).
Expanded-octet form: sulfur is 6 − 0 − ½(12) = 0. Each oxygen is 6 − 4 − ½(4) = 0.
One strict-octet contributor: sulfur is 6 − 0 − ½(8) = +2. The double-bonded oxygen is 6 − 4 − ½(4) = 0. Each single-bonded oxygen is 6 − 6 − ½(2) = −1.
The charges in each strict-octet contributor add to 0, matching neutral SO₃.
Does SO₃ Follow the Octet Rule?
Oxygen, a period-2 element, has an octet in both representations. Sulfur is in period 3. The common three-double-bond drawing places 12 electrons around sulfur, an expanded octet. A strict-octet convention instead retains only one S=O double bond in each contributor, so sulfur has eight electrons but carries a +2 formal charge. Thus the answer depends on the Lewis convention being taught, not on a change in the SO₃ molecule.
Strict octet on S
Draw three equivalent contributors, each with one S=O double bond, two S–O single bonds, sulfur at +2, and two single-bonded oxygens at −1.
Expanded valence shell
Draw three S=O double bonds. Sulfur has 12 bonding electrons, and all formal charges are 0.
SO₃ Molecular Geometry
| Electron geometry | Trigonal planar |
|---|---|
| Molecular geometry / shape | Trigonal planar |
| VSEPR notation | AX₃ |
| Bond angle | 120° |
On the central sulfur, count electron domains the VSEPR way: each bonded neighbor is one domain (a double or triple bond still counts as one), and each lone pair is one more. SO₃ has 3 bonding domains and no lone pairs (AX₃), for 3 electron domains in total.
Those domains arrange to minimize repulsion around the sulfur, so the molecule is trigonal planar with a bond angle of 120°. With no lone pairs on the central atom, the electron geometry and the molecular geometry are the same: both trigonal planar.
SO₃ is AX₃: sulfur is the central atom and the three oxygens are three bonding domains. Multiple bonds each count as one domain in VSEPR, so the electron geometry and molecular geometry are both trigonal planar. The O–S–O bond angles are 120°.
Why Is SO₃ Trigonal planar?
There are no lone pairs on sulfur in the common expanded-octet drawing. Three bonding domains spread out evenly in one plane, with each O–S–O angle at 120°. Unlike SO₂ (AX₂E), there is no lone pair to leave a bent arrangement.
Is SO₃ Polar or Nonpolar?
SO₃ is nonpolar.
Each S–O bond is polar because oxygen is more electronegative than sulfur. In SO₃, however, the three identical bonds lie in one trigonal planar arrangement, 120° apart. Their equal bond-dipole vectors cancel, leaving no permanent molecular dipole moment. SO₃ is therefore nonpolar. This differs from bent SO₂, where the two S–O bond dipoles cannot cancel.
Three equal S=O bond dipoles point toward O. The trigonal planar shape cancels them completely — net dipole 0.
| SO₃ | Trigonal planar · Nonpolar — three equal dipoles cancel |
|---|---|
| SO₂ | Bent · Polar — dipoles do not cancel |
SO₃ Hybridization
Sulfur in SO₃ is sp²-hybridized.
In the standard VSEPR classroom model, sulfur has three bonding domains and no lone pairs. That gives trigonal planar geometry and an sp² hybridization description. The label summarizes the three-domain geometry; the π bonding and electron delocalization in sulfur trioxide require a fuller bonding model than hybridization alone.
SO₃ vs Sulfur dioxide (SO₂): What Is the Difference?
SO₃ and SO₂ are both neutral sulfur oxides with polar S–O bonds, but their electron-domain counts differ. SO₃ has three bonding domains and no central lone pair, whereas SO₂ has two bonding domains and one lone pair. That one difference changes the shape and whether the bond dipoles cancel.
| Property | SO₃ | Sulfur dioxide (SO₂) |
|---|---|---|
| Valence electrons | 24 | 18 |
| Central lone pairs (expanded form) | 0 | 1 |
| VSEPR | AX₃ | AX₂E |
| Molecular geometry | Trigonal planar | Bent |
| Bond angle | 120° | ≈120° (experimental ~119°) |
| Polarity | Nonpolar — dipoles cancel | Polar — dipoles do not cancel |
SO₃ Lewis Structure Summary
| Chemical name | Sulfur trioxide |
|---|---|
| Total valence electrons | 24 |
| Central atom | Sulfur |
| Bond type | Three S=O double bonds |
| Lone pairs on sulfur | 0 |
| Lone pairs on outer atoms | 2 on each oxygen |
| Electron geometry | Trigonal planar |
| Molecular geometry | Trigonal planar |
| VSEPR | AX₃ |
| Bond angle | 120° |
| Hybridization | sp² |
| Polarity | Nonpolar |
| Formal charge | 0 on expanded form; octet forms: S +2, O −1, O −1, O 0 |
| Resonance | 3 strict-octet resonance forms |
| Overall charge | 0 |
| Octet exception | Expanded octet on S allowed |
Try Another Lewis Structure
Enter another formula to draw its Lewis structure — bonds, lone pairs, formal charges, and the steps behind them.
Need the mass instead? Open the molar mass calculator for SO₃ →
Related Lewis Structures
Common next structures from general chemistry — each link opens that molecule so you can compare geometry, polarity, or the drawing steps.
Frequently Asked Questions
What is the Lewis structure of SO₃?
A common SO₃ Lewis structure puts sulfur in the center with three S=O double bonds and two lone pairs on each oxygen. It uses 24 valence electrons, gives sulfur an expanded octet of 12 electrons, and has formal charges of 0 on every atom.
How many valence electrons does SO₃ have?
SO₃ has 24 valence electrons: 6 from sulfur and 18 from the three oxygens (3 × 6). Both the expanded-octet drawing and the strict-octet resonance contributors use all 24 electrons.
Does SO₃ have resonance structures?
Under a strict-octet convention, SO₃ has three equivalent charge-separated resonance contributors. Each has one S=O double bond and two S–O single bonds; the double bond can be placed on any of the three oxygens.
Why does SO₃ have different Lewis structures?
The drawings use different Lewis conventions. Allowing period-3 sulfur to expand its octet gives three S=O double bonds with formal charges of 0. Enforcing an octet on sulfur gives three charge-separated resonance contributors with sulfur at +2 and two oxygens at −1.
What are the formal charges in SO₃?
In the common expanded-octet drawing, sulfur and all three oxygens have formal charge 0. In one strict-octet resonance contributor, sulfur is +2, the double-bonded oxygen is 0, and each of the two single-bonded oxygens is −1; the charges still sum to 0.
What is the molecular geometry of SO₃?
SO₃ has trigonal planar molecular geometry. Sulfur has three bonding domains and no lone pairs (VSEPR AX₃), so the three S–O positions are 120° apart in one plane.
Is SO₃ polar or nonpolar?
SO₃ is nonpolar. Its individual S–O bonds are polar, but the three identical bond dipoles are arranged symmetrically in a trigonal planar geometry and cancel.