Sulfur dioxide Lewis Structure

Sulfur Dioxide Lewis Structure: SO₂ Lewis Structure, Bent Geometry, sp² Hybridization & Polarity

Sulfur dioxide Key Facts

The sulfur dioxide Lewis structure is best drawn as O=S–O with one lone pair on sulfur and resonance between the two S–O bonds; the molecule is bent, the central sulfur is sp²-hybridized, the O–S–O angle is about 119°, and because the bent shape prevents dipole cancellation, SO₂ is polar.

Figure 1: Lewis Structure of Sulfur dioxide (SO₂)

Chemical Formula	SO₂
CAS Number	7446-09-5
Total Valence Electrons	18
Lewis Structure	O=S–O with one S lone pair (resonance)
Electron-Domain Geometry	Trigonal planar
Molecular Geometry	Bent / angular
Hybridization Model	sp² (central sulfur)
O–S–O Bond Angle	≈ 119°
Polarity	Polar molecule

PubChem and NIST identify sulfur dioxide as SO ₂ with CAS 7446-09-5; PubChem lists a molecular weight of approximately 64.07 g/mol. View CAS 7446-09-5 dictionary details on Guidechem →

Applications of Sulfur dioxide (CAS 7446-09-5)

Sulfur dioxide is a workhorse of industrial sulfur chemistry. It is the key intermediate in the Contact process for sulfuric acid: SO₂ is catalytically oxidized to SO₃, which is then absorbed to make the world's most-produced industrial chemical by mass. Large volumes are also generated and recovered during the roasting of metal sulfide ores.

Beyond acid manufacture, SO₂ is an important food preservative and antioxidant (E220), used to inhibit microbial growth and enzymatic browning in dried fruit, wine and some fruit products. In pulp and paper, SO₂ and its derivatives (sulfite/bisulfite) are central to the sulfite pulping process. It also finds use as a reducing agent, a bleaching agent for textiles and a historical refrigerant.

Sulfuric Acid Production Intermediate oxidized to SO₃ in the Contact process; the basis of global sulfuric acid supply.

Pulp & Paper Sulfite pulping and bleaching chemistry for wood and recycled fiber.

Food Preservative (E220) Antioxidant and antimicrobial agent in dried fruit, wine and fruit preparations, within regulatory limits.

Procurement Note "Sulfur dioxide" quality is defined by purity, acidity, moisture and trace-metal limits, which differ sharply between food/pharmaceutical and industrial grades. Because SO₂ is toxic, corrosive and a regulated air pollutant, procurement must also address cylinder/bulk handling, emission controls and local compliance. Always use CAS 7446-09-5 as the identifiers when screening suppliers.

Step 1 - Valence Electrons & Central Atom

To draw the SO₂ Lewis structure, first count valence electrons. Sulfur is in Group 16 and contributes 6 valence electrons. Each oxygen is also in Group 16 and contributes 6 valence electrons.

Valence-electron count

Total valence electrons = 6 from S + 6 from O + 6 from O = 18 electrons.

Step 2 - Lewis Structure & Electron Distribution

Write the skeleton O – S – O. Two single S–O bonds consume 4 electrons, leaving 14. Placing lone pairs gives each oxygen three lone pairs (12 electrons) and sulfur one lone pair (2 electrons), but sulfur then has only 6 electrons around it and the formal charges are unfavorable.

Promoting one S–O single bond to a double bond gives O=S–O. The double bond uses 4 electrons, each oxygen now has two lone pairs (or one does, with the other bearing the double bond), sulfur keeps one lone pair, and all atoms achieve octets. The two S–O bonds are equivalent by resonance, so the true structure is a resonance hybrid with two equal S–O bonds of partial double-bond character.

O = S – O

Sulfur carries one lone pair; two equivalent S–O bonds by resonance

3 electron domains around S (2 bonds + 1 lone pair)

Octet & Resonance in SO₂ Sulfur (a third-row element) comfortably accommodates the bonding in SO₂, and the octet rule is satisfied on all three atoms in each resonance contributor. Because the two S–O bonds are symmetry-equivalent, neither single-bond nor double-bond placement is uniquely correct; the molecule is best represented by two resonance structures with the negative charge delocalized, giving both S–O bonds equal length and strength.

Step 3 - VSEPR Theory & Geometry

The Lewis structure gives three electron domains around sulfur: two bonding domains (the S–O bonds) and one lone pair. Three electron domains adopt a trigonal-planar electron-domain geometry to minimize repulsion.

The lone pair occupies more space than a bonding pair, pushing the two S–O bonds closer together. As a result, the molecular geometry is bent rather than trigonal planar, and the O–S–O bond angle is compressed slightly below the ideal 120°, to about 119°.

Repulsion hierarchy

The useful VSEPR ordering is: lone pair–bonding pair repulsion > bonding pair–bonding pair repulsion. The sulfur lone pair therefore repels the two S–O bonding domains more strongly than they repel each other, bending the molecule and slightly narrowing the bond angle from 120° to roughly 119°.

Hybridization & Formal Charge

In the localized-bond model, the central sulfur in SO₂ is described as sp²-hybridized: three electron domains correspond to three approximately trigonal-planar hybrid orbitals. Two form σ bonds to oxygen and one holds the lone pair; the remaining unhybridized p orbital participates in the π system that gives the S–O bonds their resonance (partial double-bond) character.

Advanced descriptions treat the π system as delocalized over S and both O atoms, consistent with the observed equal S–O bond lengths.

Formal-charge check

Formal charge = valence − nonbonding − ½ × bonding. In the major O=S–O contributor: sulfur 6 − 2 − ½×6 = +1; double-bonded oxygen 6 − 4 − ½×4 = 0; single-bonded oxygen 6 − 6 − ½×2 = −1. The resonance average gives both oxygens an equal partial negative character and sulfur a partial positive charge.

Modeling Note sp² hybridization is a convenient model for predicting the trigonal-planar electron-domain arrangement. Sulfur can use d-orbital-involved or hypervalent descriptions in older textbooks; modern MO theory describes the bonding through delocalized π bonding without requiring expanded octets as literal d-orbital occupation.

Is Sulfur dioxide Polar or Nonpolar?

Sulfur dioxide is polar. The S–O bonds are polar because oxygen is more electronegative than sulfur, and the molecule is bent. The two bond dipoles therefore do not cancel; their vector sum points roughly toward the oxygen side of the molecule, giving SO₂ a measurable permanent dipole.

This polarity, together with the lone pair on sulfur, makes SO₂ a good Lewis base at sulfur and a good Lewis acid at oxygen, underpinning its reactivity as both a reducing agent and a ligand in coordination chemistry.

Expert Quote — Structural Chemistry Perspective

"SO₂ is polar not merely because the S–O bonds are polar, but because the bent geometry breaks dipole cancellation. The sulfur lone pair defines the electron-domain arrangement, and it is that lone pair which forces the molecule away from linearity and preserves a net dipole."

Interpretation consistent with VSEPR and molecular-symmetry treatments of bent triatomic AX₂E molecules.

The polarity of SO₂ also explains its high solubility in water and its ability to dissolve to form sulfurous acid (H₂SO₃) and bisulfite/bisulfite species — the chemistry behind its use as a food preservative and its role as an acid-rain precursor in the atmosphere.

Structural Comparison Table

Species	Formula	Central Atom	Electron Domains	Molecular Geometry	Polarity
Sulfur dioxide	SO₂	S	3	Bent	Polar
Sulfur trioxide	SO₃	S	3	Trigonal planar	Nonpolar
Carbon dioxide	CO₂	C	2	Linear	Nonpolar
Ozone	O₃	O	3	Bent	Polar
Water	H₂O	O	4	Bent	Polar

Practice question Draw both resonance structures of SO₂ and assign formal charges. Then explain why the bent geometry (rather than the polar bonds alone) is what makes the molecule polar, and compare it to CO₂.

Looking for industrial bulk or specified-grade supplies of Sulfur dioxide (CAS 7446-09-5)? Browse Supplier on Guidechem →

FAQs

What is the Lewis structure of SO₂?

O=S–O with one sulfur lone pair, resonance-delocalized. The two S–O bonds are equivalent by resonance.

Is SO₂ polar or nonpolar?

SO₂ is polar. Its bent geometry prevents the two polar S–O bond dipoles from canceling.

What is the bond angle in SO₂?

The O–S–O angle is about 119°. Three electron domains give a near-trigonal-planar arrangement compressed slightly by the lone pair.

What is the hybridization of sulfur in SO₂?

Sulfur is sp²-hybridized. Three electron domains (2 bonds + 1 lone pair) correspond to three trigonal-planar hybrids.

What is the CAS number of sulfur dioxide?

The CAS number is 7446-09-5. PubChem (CID 1119) and NIST identify this with SO₂.

References

PubChem, Sulfur Dioxide, CID 1119. Molecular formula, CAS number and molecular weight (≈64.07 g/mol).
NIST Chemistry WebBook, SRD 69, Sulfur Dioxide. Formula, CAS Registry Number, thermochemical and spectroscopic data.
IUPAC Gold Book, "resonance" and "lone pair." Definitions underpinning the resonance-hybrid description of SO₂.
Greenwood, N. N. & Earnshaw, A., Chemistry of the Elements. Industrial sulfur chemistry, the Contact process and SO₂ behavior.
WHO Food Additives Series, Sulfur Dioxide (E220). Regulatory and safety context for food-grade use.

Technical scope: Lewis structures, VSEPR and hybridization are model-based descriptions for chemical communication; real bonding in SO₂ is better described with molecular orbitals and delocalized π bonding. Commercial SO₂ grades differ in purity and impurity profile, and CAS 7446-09-5 identifies the substance, not a specific grade or emission-compliance status.