What Shape Does SO3 Have?

SO3 has a Trigonal planar molecular geometry (AX3, trigonal planar) with a bond angle of 120°. The central sulfur carries 3 bonding domain(s) and 0 lone pair(s) — 3 electron domains in total. Remove so2's lone pair and the three bonding domains spread out to perfect planar symmetry.

Hybridisation is conventionally described as sp2. Polarity: Nonpolar (symmetric cancellation of the three S–O dipoles).

1. Molecular Identity and Key Parameters

Molar mass is the average value rounded for procurement use. Hybridisation labels are the conventional VSEPR-level description; orbital treatment becomes more nuanced at higher levels of theory.

2. Drawing the Lewis Structure of SO3, Step by Step

1. Count valence electrons: 6 (S) + 3×6 (O) = 24 — a total of 24 electrons (12 pairs) must be placed.
2. Build the skeleton: place the sulfur at the centre and attach the terminal atoms with single bonds; hydrogen is never a central atom. Each single bond consumes 2 electrons.
3. Complete terminal octets: distribute the remaining electrons as lone pairs on the terminal atoms first (hydrogens already carry their duet from the single bond).
4. Satisfy the central atom: Three S–O bonding domains around sulfur with no lone pair; the double-bond character is delocalised over all three positions.
5. Check formal charges and resonance: In the three-double-bond form every atom is formal-charge zero. Resonance: Three equivalent S–O bonds (delocalised π system).
6. Verify: Sulfur is hypervalent in the double-bonded description. Apply the standard formal-charge check (valence − nonbonding − half of bonding electrons) to confirm the drawn form.

A worked diagram of this molecule is kept in the companion guide: SO3 Lewis structure.

3. VSEPR Analysis: Why SO3 Is trigonal planar

VSEPR theory counts electron domains — every σ bond counts once regardless of bond order, and every lone pair counts once. SO3 has 3 such domains around the sulfur: 3 bonding and 0 lone. 3 domains correspond to a trigonal planar electron-domain arrangement; with no lone pair to hide the observed molecular shape is trigonal planar (AX3).

Bond angle: 120° because remove SO2's lone pair and the three bonding domains spread out to perfect planar symmetry.

Polarity: Nonpolar (symmetric cancellation of the three S–O dipoles).

Bond-order note: Three equivalent S–O bonds (delocalised π system).

4. The Most Common Mistake When Drawing SO3

5. Why the Geometry Matters in Practice

Sulfonation of detergents, dyes and pharmaceutical intermediates; shipped as aged oleum or controlled SO3 generators because the monomer polymerises readily.

Structure-level identity is exactly what a purchase order has to fix: the same molecular formula can hide isomers, hydrates or salt forms with different handling profiles. Before ordering, match CAS (7446-11-9), assay, water content and container rating against the datasheet, and request the COA batch that will ship.

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6. Quick FAQs on SO3

7. References

1. IUPAC / Gillespie, Assessment of the VSEPR model. Basis of AXE notation and lone-pair ordering. VSEPR entry
2. PubChem compound record for Sulfur Trioxide (SO3). Registry number, formula and computed descriptors. PubChem 7446-11-9
3. Companion Lewis-structure guide on this site. Worked diagram for SO3. Lewis structure SO3