SO₃ has 24 valence electrons and trigonal planar geometry. The generator shows the common expanded-octet form with three S=O bonds and zero formal charges, while some courses discuss charge-separated resonance contributors.
Common expanded-octet representation; trigonal planar atom arrangement
Valence electrons24
Central atomSulfur
GeometryTrigonal planar
Bond angle120°
How to draw SO3
Count 24 electrons.
Sulfur contributes 6 and three oxygens contribute 18.
Connect sulfur to three oxygens.
Begin with three S–O single bonds.
Complete oxygen octets.
This creates a charge-separated octet-only contributor.
Apply the convention used by your course.
In the common expanded-octet form, oxygen lone pairs become three S=O bonds and formal charges become zero.
Check the geometry.
Three bonding domains and no sulfur lone pair give trigonal planar geometry.
Why drawings differ
Lewis structures are simplified electron-accounting models. Expanded-octet and charge-separated contributors emphasize different bookkeeping choices; neither should be interpreted as a complete quantum-mechanical bonding description.
Geometry
All three oxygen atoms occupy equivalent positions around sulfur in a plane. The ideal O–S–O angle is 120°.
Common mistakes
Giving sulfur a lone pair and calling SO3 trigonal pyramidal.
Confusing neutral SO3 with sulfate, SO4²−.
Using only 18 electrons.
Presenting one Lewis convention as a literal picture of localized bonds.