1
Parse the molecular formula
SO3 -> S, O, O, O
The molecule contains one sulfur atom and three oxygen atoms.
Lewis Structure
SO3 Lewis Structure
Sulfur trioxide
A common SO3 Lewis drawing has sulfur in the center with three S=O double bonds.
Use the diagram and steps below to check the sulfur trioxide Lewis structure, SO3 Lewis dot structure, valence electrons, formal charges, and molecular shape.
SO3 can also be discussed using resonance. Check your course convention for expanded octet examples.
SO3
Sulfur trioxide
Valence electrons24
Central atomS
GeometryTrigonal planar
Bond angleAbout 120 deg
PolarityNonpolar
Electron geometryTrigonal planar
VSEPR Geometry
Molecular geometry
Trigonal planar
Electron geometry
Trigonal planar
Bond angle
About 120 deg
Polarity
Nonpolar
Valence electrons
24
Bonding / lone pairs
3 / 0 around S in the shown form
Central atom
S
Electron regions
3
How to Draw SO3: Step-by-Step
2
Count total valence electrons
6 + 6 + 6 + 6 = 24
Sulfur contributes 6 electrons, and three oxygen atoms contribute 18.
3
Choose the central atom
Central atom: S
Sulfur is placed in the center with three terminal oxygen atoms.
4
Draw the common form
Three S=O double bonds
The shown classroom form uses an expanded octet on sulfur.
5
Place lone pairs
Two lone pairs on each O
Each oxygen has two lone pairs, and the shown form has zero formal charges.
6
Final answer
The SO3 Lewis structure shown here has sulfur in the center with three S=O double bonds and two lone pairs on each oxygen.
Formal Charges
| Atom | Formal charge |
|---|---|
| S | 0 in the shown form |
| O | 0 in the shown form |
Common Mistakes
- SO3 is trigonal planar, while SO2 is bent.
- Some courses prefer resonance forms with formal charges; note the convention being used.
FAQ
What is the Lewis structure for SO3?
The SO3 Lewis structure shown here has sulfur in the center with three S=O double bonds and two lone pairs on each oxygen.
What is the Lewis dot structure for SO3?
The Lewis dot structure for SO3 is the electron-dot drawing shown above. The SO3 Lewis structure shown here has sulfur in the center with three S=O double bonds and two lone pairs on each oxygen.
How many valence electrons does SO3 have?
SO3 has 24 total valence electrons in this Lewis structure.
What is the molecular geometry of SO3?
SO3 has trigonal planar molecular geometry and trigonal planar electron geometry.
Is SO3 polar or nonpolar?
SO3 is nonpolar in the Lewis structure shown here. Its molecular geometry is Trigonal planar.
References
Method notes are aligned with standard Lewis structure steps: count valence electrons, draw a skeleton, distribute electrons, form multiple bonds when needed, and check formal charges and resonance.
OpenStax Chemistry 2e: Lewis Symbols and Structures and Formal Charges and Resonance.