Sulfur trioxide Lewis Structure

Sulfur trioxide Lewis Structure: O₃S Lewis Structure, trigonal planar Geometry, sp2 Hybridization & Polarity

Sulfur trioxide Key Facts

The Sulfur trioxide Lewis structure is S–O double bond, S–O double bond and S–O double bond; the central S has 3 electron domains (0 lone pairs), giving a trigonal planar electron-domain arrangement and a trigonal planar molecular geometry with a 120° bond angle. S is best described as sp2-hybridized, and Sulfur trioxide is nonpolar because the bond dipoles cancel by molecular symmetry.

Figure 1: Lewis Structure of Sulfur trioxide (O₃S)

Chemical Formula	O₃S
CAS Number	7446-11-9
Total Valence Electrons	24
Lewis Structure	S–O double bond, S–O double bond and S–O double bond
Electron-Domain Geometry	trigonal planar
Molecular Geometry	trigonal planar
Hybridization Model	sp2 (central S)
Bond Angle	120°
Polarity	Nonpolar molecule

PubChem (CID 24682) and NIST list Sulfur trioxide as O ₃S with CAS 7446-11-9; molecular weight ≈ 80.07 g/mol. View CAS 7446-11-9 dictionary details on Guidechem →

Applications of Sulfur trioxide (CAS 7446-11-9)

The S oxide Sulfur trioxide is a key industrial intermediate. Nonmetal oxides such as SO₃ and P₄O₁₀ are acid anhydrides for sulfuric and phosphoric acid manufacture, while metal/semimetal oxides are ceramics, refractories, catalysts and pigment precursors.

Acid Anhydride / Intermediate Gateway to the corresponding oxoacid and its salts (fertilizers, detergents).

Materials & Catalysis Oxide ceramics, refractories, catalysts and pigment feedstocks.

Process Gas / Reagent Oxidizing or reactive reagent in inorganic and environmental processing.

Procurement Note "Sulfur trioxide" is supplied in multiple commercial grades and package sizes. Specify purity, grade, moisture/impurity limits, package (cylinder, drum or bulk) and intended process, and use CAS 7446-11-9 as the primary identifier when screening suppliers; always confirm regional availability and safety documentation (SDS).

Step 1 - Valence Electrons & Central Atom

To draw the Sulfur trioxide Lewis structure we first count the total valence electrons. O₃S contains 3×6 (O) + 6 (S); the net charge of 0 gives a total of 24 valence electrons to place.

The central atom is S. It is the least electronegative atom capable of forming multiple bonds and is surrounded by the more electronegative terminal atom(s); hydrogen, when present, always occupies a terminal position. Electronegativity considerations place the electron density toward the terminal atom(s).

Valence-electron count

Total valence electrons = 3×6 (O) + 6 (S) = 24 electrons.

Step 2 - Lewis Structure & Electron Distribution

Place S at the center and connect each surrounding atom with a single bond, complete octets on the terminal atoms, and place any remaining electrons on the central atom. The resulting electron distribution is: S–O double bond, S–O double bond and S–O double bond; no lone pairs on S.

Counting bonding and nonbonding electrons around S therefore satisfies the octet/expanded-octet requirement for the drawn structure.

This species is best represented by resonance structures; the π bonding is delocalized over the equivalent terminal atom(s), so all S–terminals bonds are equivalent in the resonance hybrid.

S=O S=O S=O

S: 0 lone pairs, 3 bonding domains

3 bonding domains + 0 lone pairs = 3 electron domains

Octet Rule: Why S Has Eight Electrons S forms 3 bond(s) and retains 0 nonbonding electron(s), reaching an octet. No expanded octet is required for this second-row (or octet-satisfying) central atom.

Step 3 - VSEPR Theory & Geometry

VSEPR theory states that the 3 electron domains around S arrange to minimize mutual repulsion, giving an trigonal planar electron-domain geometry.

Because 0 of these domains are lone pair(s) — and lone pairs repel more strongly than bonding pairs — the observed molecular geometry is trigonal planar; the S bond angle is set to 120° rather than the ideal value for the electron-domain geometry.

Repulsion hierarchy

A useful VSEPR approximation is: lone pair–lone pair repulsion > lone pair–bonding pair repulsion > bonding pair–bonding pair repulsion. With 0 lone pair(s) on S, the lone-pair domains occupy positions that minimize overall repulsion while pushing the bonding domains (and therefore the bonded atoms) closer together.

Hybridization & Formal Charge

In the conventional localized-bond model, S uses sp2 hybrid orbitals to form the 3 equivalent electron-domain directions (3 bonding + 0 lone pair).

Formal-charge check

Formal charge on central S = V − N − ½B = 6 − 0 − ½×12 = 0. The preferred Lewis structure carries zero formal charge on the central atom (and on the terminals), so it is the dominant contributor.

Modeling Note Modeling Note: sp2 is a compact model for the 3 electron-domain directions around S; full quantum-chemical treatments use molecular orbitals.

Is Sulfur trioxide Polar or Nonpolar?

Sulfur trioxide is nonpolar. Although individual S–terminal bonds are polar when the atoms differ in electronegativity, the trigonal planar geometry makes their vector sum zero, so there is no net molecular dipole.

Expert Quote — Structural Chemistry Perspective

A common misconception is that polar bonds guarantee a polar molecule. Sulfur trioxide shows the opposite: identical bond dipoles arranged with trigonal planar symmetry cancel exactly, yielding a nonpolar species.

Interpretation consistent with standard VSEPR / molecular-geometry references.

Its nonpolar character governs solvation and intermolecular behavior; specify grade and handling when sourcing.

Structural Comparison Table

Species	Formula	Central Atom	Electron Domains	Molecular Geometry	Polarity
Sulfur trioxide	O₃S	S	3	trigonal planar	Nonpolar
Sulfur dioxide	SO₂	S	3	bent	Polar
Sulfur hexafluoride	F₆S	S	6	octahedral	Nonpolar
Sulfur tetrafluoride	F₄S	S	5	seesaw	Polar
Sulfate	O₄S	S	4	tetrahedral	Nonpolar
Sulfite	O₃S	S	4	trigonal pyramidal	Polar

Practice question Starting from the 24 valence electrons of O₃S, draw the Lewis structure and confirm the central S atom is sp2-hybridized with trigonal planar geometry. What happens to the bond angle if one terminal atom is replaced by a more electronegative substituent?

Looking for industrial bulk or specified-grade supplies of Sulfur trioxide (CAS 7446-11-9)? Browse Supplier on Guidechem →

FAQs

Is O₃S polar or nonpolar?

Sulfur trioxide is nonpolar. Its trigonal planar symmetry cancels the bond dipoles exactly.

What is the Lewis structure of Sulfur trioxide?

S–O double bond, S–O double bond and S–O double bond; no lone pairs on S. It uses all 24 valence electrons and the preferred structure carries zero formal charge on the central atom.

What is the bond angle in Sulfur trioxide?

The S angle is about 120°. Lone-pair repulsion compresses the angle from the ideal trigonal planar value.

What is the hybridization of S in Sulfur trioxide?

The conventional model is sp2. Sulfur trioxide has 3 electron domains around the central S.

What is the CAS number of Sulfur trioxide?

The CAS number is 7446-11-9. PubChem (CID 24682) and NIST identify this with O₃S; use it to locate suppliers and database records.

References

PubChem CID 24682, Sulfur trioxide. Molecular formula, CAS number, molecular weight and chemical identity.
NIST Chemistry WebBook, SRD 69, Sulfur trioxide. Formula, CAS Registry Number and thermochemical / spectroscopic data.
IUPAC Gold Book. Definitions for "lone pair", "VSEPR", "hybridization" and "formal charge".
Atkins, P. & de Paula, J., Physical Chemistry. VSEPR, molecular geometry and dipole moments.
Guidechem supplier & dictionary records. Commercial sourcing and CAS-linked property pages for Sulfur trioxide (CAS 7446-11-9).

Technical scope: Lewis structures, VSEPR and hybridization are model-based descriptions for chemical communication and structure prediction; actual molecular electronic structure is more accurately represented by molecular-orbital and quantum-chemical methods. Commercial grades vary substantially by supplier and intended use; CAS 7446-11-9 identifies the substance, not a universal purity specification.

Make Guideview Preferred Source on Google

Sulfur trioxide Lewis Structure