SO₂ Lewis Structure
Sulfur dioxide has 18 valence electrons. Sulfur is the central atom and the molecule is bent. Textbooks commonly draw SO₂ either as O=S=O with an expanded octet on sulfur, or as a pair of charge-separated resonance forms that keep sulfur to eight electrons. Both are valid; they differ in formal charges and how the octet rule is applied, but they share the same bent geometry and polarity.
| Total valence electrons | 18 |
|---|---|
| Central atom | Sulfur (S) |
| Bonds | Two S=O double bonds |
| Lone pairs on S | 1 |
| Lone pairs on O | 2 |
| Electron geometry | Trigonal planar |
| Molecular geometry | Bent |
| VSEPR | AX₂E |
| Bond angle | ≈120° |
| Polarity | Polar |
| S hybridization | sp² |
| Resonance | Yes |
| Formal charges | 0 (expanded form) |
Important
Why do some SO₂ Lewis structures look different? SO₂ is commonly drawn either as an expanded-octet structure with two S=O bonds (formal charges all zero) or as charge-separated resonance contributors that keep sulfur to an octet. Both appear in textbooks. Their formal charges differ; the bent shape and polarity do not. Use the convention your course requires.
How to Draw the SO₂ Lewis Structure
Draw SO₂ step by step. Count the valence electrons, choose the central atom, connect the atoms with single bonds, finish the outer octets, then form multiple bonds if an atom is still short of a full shell.
Step 1Count the valence electrons
Sulfur brings 6 valence electrons and each oxygen brings 6. With two oxygens that is 6 + 12 = 18 electrons to place — and SO₂ is neutral, so there is no charge to add or remove. Every valid Lewis drawing of SO₂ must use all 18.
Atom Count Valence e⁻ Total S 1 6 6 O 2 6 12 Total 18 Step 2Place sulfur in the center
Sulfur is less electronegative than oxygen and can form more than one bond, so it sits in the middle. Connect the atoms with single bonds first: O—S—O. Those two bonds use 4 of the 18 valence electrons, leaving 14 to assign as lone pairs or multiple bonds.
Step 3Complete the outer octets
Give each oxygen enough lone pairs to reach eight electrons around it, then put any leftover electrons on sulfur as a lone pair. At this stage the skeleton still has only single S–O bonds. The next step forms multiple bonds — or a charge-separated resonance pair — depending on whether sulfur is allowed an expanded octet.
Step 4Form S=O bonds — two classroom conventions
Do not assume the only answer is O=S=O. If sulfur may expand its octet (period 3), move one lone pair from each oxygen into the S–O links to make two double bonds: O=S=O with formal charges all 0 and 10 electrons on sulfur. If every atom must keep a strict octet, form only one S=O double bond and leave the other S–O single, producing the charge-separated pair O=S⁺–O⁻ ↔ ⁻O–S⁺=O. Both use 18 electrons, and both are taught in general chemistry.
Step 5Check formal charges and the electron count
Formal charge = valence e⁻ − nonbonding e⁻ − ½(bonding e⁻). On expanded O=S=O every atom scores 0. On one octet contributor the double-bonded oxygen is 0, sulfur is +1, and the single-bonded oxygen is −1 (the other contributor swaps the oxygens). The formal charges always sum to 0 for neutral SO₂, and the atom electron counts must still total 18 valence electrons placed.
Electron counts check out (S 10 · O 8 · O 8): hydrogen targets 2 electrons, other atoms an octet unless noted. All 18 valence electrons are placed.
SO₂ Valence Electrons
Sulfur contributes 6 valence electrons, and 2 × oxygens contribute 2 × 6 = 12. Added together, SO₂ starts with 18 valence electrons to place, and as a neutral molecule there is no charge to add or remove. Of those, 8 fill the bonds and 10 sit as lone pairs (2 on each oxygen).
| sulfur: 6 valence electrons |
| 2 × oxygen: 2 × 6 = 12 |
| Total: 6 + 12 = 18 |
SO₂ Resonance Structures
SO₂ still involves resonance even when many courses prefer a single O=S=O drawing. The octet-preserving contributors put a double bond on one oxygen and a single bond plus a negative formal charge on the other, with sulfur at +1. Those two drawings are two ways of writing the same hybrid — not two different molecules, and not a frozen ‘left oxygen single, right oxygen double’ structure. In the real molecule both oxygens are chemically equivalent and both S–O bonds share the same average bond order. Expanded-octet O=S=O captures that hybrid in one picture: two equal S=O bonds and formal charge 0 on every atom. A lone O=S⁺–O⁻ sketch is only one contributor; the resonance arrow to ⁻O–S⁺=O is what shows the oxygens are equivalent.
Octet-preserving resonance contributors
Each form keeps eight electrons on sulfur. The multiple bond sits on one oxygen or the other; the single-bonded oxygen carries −1 and S carries +1. With the resonance arrow between them, both oxygens stay equivalent — the hybrid is not permanently single-bonded on one side and double-bonded on the other.
Expanded-octet representation
If sulfur may expand its valence shell, one drawing is enough: two equal double bonds, formal charge 0 on every atom, and 10 electrons around S. The two oxygens remain equivalent without drawing a second contributor.
Which SO₂ Lewis Structure Is Correct?
There is no single wrong drawing of SO₂. Expanded O=S=O is popular because every formal charge is zero and both S–O bonds look the same. The charge-separated pair is useful when every atom, including sulfur, must keep an octet. More advanced courses often describe the bonding as a resonance hybrid with partial double-bond character on both sides. Use the convention your course requires; either way, treat the two oxygens as equivalent and the molecular geometry as bent.
| Representation | S octet | Formal charges | Use |
|---|---|---|---|
| O=S=O | Expanded octet | 0, 0, 0 | Common textbook representation |
| O=S–O⁻ ↔ ⁻O–S=O | Octet on S | Charge separation (+1 / −1) | Resonance / octet-focused explanation |
SO₂ Lewis Structure with Formal Charges
Formula
Formal charge = valence e⁻ − nonbonding e⁻ − ½ × bonding e⁻
Expanded-octet structure
S: 6 − 2 − ½(8) = 0 · O: 6 − 4 − ½(4) = 0 · O: 6 − 4 − ½(4) = 0
| Atom | Calculation | Formal charge |
|---|---|---|
| S | 6 − 2 − ½(8) = 0 | 0 |
| O | 6 − 4 − ½(4) = 0 | 0 |
| O | 6 − 4 − ½(4) = 0 | 0 |
Octet resonance contributor
One contributor is shown; the other only swaps which outer atom holds the extra bonding. S: 6 − 2 − ½(6) = +1 · O: 6 − 6 − ½(2) = −1 · O: 6 − 4 − ½(4) = 0
| Atom | Calculation | Formal charge |
|---|---|---|
| S | 6 − 2 − ½(6) = +1 | +1 |
| O | 6 − 6 − ½(2) = −1 | −1 |
| O | 6 − 4 − ½(4) = 0 | 0 |
Formal charge is a counting tool, not a measured partial charge. For each atom: formal charge = valence electrons − nonbonding electrons − ½(bonding electrons).
Expanded O=S=O: sulfur has 1 lone pair (2 nonbonding e⁻) and two double bonds (8 bonding e⁻), so 6 − 2 − ½(8) = 0. Each oxygen has 2 lone pairs (4 nonbonding) and one double bond (4 bonding), so 6 − 4 − ½(4) = 0. Every atom is zero.
One octet contributor O=S⁺–O⁻: the double-bonded oxygen is still 6 − 4 − ½(4) = 0. Sulfur has 1 lone pair and one double bond plus one single bond (6 bonding e⁻), so 6 − 2 − ½(6) = +1. The single-bonded oxygen has 3 lone pairs (6 nonbonding) and one single bond (2 bonding), so 6 − 6 − ½(2) = −1. The other contributor only swaps which oxygen is which. Formal charges always sum to 0 for neutral SO₂. Expanded O=S=O keeps every formal charge at zero; the octet pair accepts a +1/−1 split so sulfur can stay at eight electrons.
Does SO₂ Follow the Octet Rule?
Sulfur is in period 3 and can use an expanded valence shell, so O=S=O with 10 electrons around sulfur is chemically allowed in the usual general-chemistry rules. If you refuse any expanded octet and demand eight electrons on sulfur, you cannot keep two double bonds and zero formal charges at once — you land on the charge-separated resonance contributors instead. Oxygen, a period-2 element, never expands; both conventions keep each oxygen at eight electrons. The “does SO₂ follow the octet rule?” answer is therefore: yes for oxygen always; for sulfur it depends on which valid representation you draw.
Strict octet on S
Draw the charge-separated pair O=S⁺–O⁻ ↔ ⁻O–S⁺=O.
Expanded valence shell
Draw O=S=O with 10 electrons on sulfur and formal charges of 0.
SO₂ Molecular Geometry
| Electron geometry | Trigonal planar |
|---|---|
| Molecular geometry / shape | Bent |
| VSEPR notation | AX₂E |
| Bond angle | ≈120° |
On the central sulfur, count electron domains the VSEPR way: each bonded neighbor is one domain (a double or triple bond still counts as one), and each lone pair is one more. SO₂ has 2 bonding domains and 1 lone pair (AX₂E), for 3 electron domains in total.
Those domains arrange to minimize repulsion around the sulfur, so O═S═O is bent with a bond angle of ≈120°. Lone pairs on sulfur take space but do not count in the shape name, so the molecular geometry (bent) differs from the electron geometry (trigonal planar).
VSEPR notation is AX₂E: A is sulfur, X₂ are the two oxygen bonding domains, and E is the lone pair on sulfur. The electron geometry is trigonal planar because three domains need about 120° of space; the molecular geometry is bent (sometimes called angular) because only the two oxygens count in the shape. Classroom VSEPR therefore quotes ≈120°; the experimental gas-phase O–S–O angle is about 119°.
Why Is SO₂ Bent?
SO₂ is not linear like CO₂. Carbon dioxide is AX₂ with no lone pairs on carbon, so the two double bonds sit 180° apart. Sulfur dioxide is AX₂E: sulfur has two bonding regions plus one lone pair. Those three electron domains arrange in a trigonal planar pattern, and the lone pair occupies one corner, so the two S–O bonds bend. Name the electron geometry from all domains first; name the molecular geometry from the atoms only — that is why the shape is bent while the electron geometry stays trigonal planar.
Is SO₂ Polar or Nonpolar?
SO₂ is polar.
Each S–O bond is polar: oxygen (electronegativity 3.44) pulls shared electrons harder than sulfur (2.58), so every bond dipole points toward oxygen. Bond polarity alone does not decide the molecule. If SO₂ were linear like CO₂, the two equal dipoles would sit 180° apart and cancel. Sulfur keeps a lone pair (VSEPR AX₂E), so the shape is bent at about 120° instead. In that bent geometry the bond dipoles are no longer opposite on one line — they share a component toward the oxygen side of the bend, those components add, and a net molecular dipole remains. SO₂ is therefore polar. CO₂ has the same kind of polar X=O bonds but linear AX₂ geometry, so its dipoles cancel and the molecule is nonpolar. Geometry, not bond polarity alone, makes the difference.
SO₂ Hybridization
Sulfur in SO₂ is sp²-hybridized.
The central sulfur has three electron domains — two S–O bonding regions plus one lone pair — so it mixes one 3s orbital with two 3p orbitals into three sp² hybrids. Two hybrids form the σ bonds to oxygen; the third holds the lone pair. That arrangement matches the trigonal planar electron geometry and the bent molecular shape. Any π bonding between sulfur and oxygen uses the remaining p orbital on sulfur.
SO₂ Lewis Structure Summary
| Chemical name | Sulfur dioxide |
|---|---|
| Total valence electrons | 18 |
| Central atom | Sulfur |
| Structure | O═S═O |
| Bond type | Two S=O double bonds |
| Lone pairs on sulfur | 1 |
| Lone pairs on outer atoms | 2 on each oxygen |
| Electron geometry | Trigonal planar |
| Molecular geometry | Bent |
| VSEPR | AX₂E |
| Bond angle | ≈120° |
| Hybridization | sp² |
| Polarity | Polar |
| Formal charge | 0 on expanded form; ±1 on octet forms |
| Resonance | Yes |
| Overall charge | 0 |
| Octet exception | Expanded octet on S allowed |
Try Another Lewis Structure
Enter another formula to draw its Lewis structure — bonds, lone pairs, formal charges, and the steps behind them.
Related Lewis Structures
Common next structures from general chemistry — each link opens that molecule so you can compare geometry, polarity, or the drawing steps.
Frequently Asked Questions
What is the Lewis structure of SO₂?
SO₂ is usually drawn with sulfur in the center bonded to two oxygens. The common textbook form is O=S=O with one lone pair on sulfur and two on each oxygen (18 valence electrons, formal charges all 0, expanded octet on S). An equally valid octet-focused set is the resonance pair O=S⁺–O⁻ ↔ ⁻O–S⁺=O.
How many valence electrons are in SO₂?
SO₂ has 18 valence electrons: 6 from sulfur and 6 from each oxygen (2 × 6 = 12). Those electrons fill the S–O bonds and the lone pairs in every valid drawing of the molecule.
Does SO₂ have resonance structures?
Yes. The octet-preserving drawings are a two-form resonance pair with the double bond on one oxygen or the other. Many texts also show a single expanded-octet O=S=O structure that represents the averaged bonding with zero formal charges.
Why does SO₂ have different Lewis structures?
The drawings differ by teaching convention. Allowing sulfur an expanded octet gives O=S=O with formal charges of zero. Enforcing an octet on sulfur gives charge-separated resonance forms. Both place 18 electrons and both lead to the same bent molecular geometry.
Does SO₂ obey the octet rule?
Oxygen always keeps an octet in the usual drawings. Sulfur can be drawn with eight electrons (charge-separated resonance forms) or with an expanded octet of 10 electrons in O=S=O. Period-3 sulfur is allowed either description in general chemistry.
What is the molecular geometry of SO₂?
SO₂ has a bent molecular geometry. Sulfur has two bonding domains and one lone pair (VSEPR AX₂E), so the electron geometry is trigonal planar and the visible shape is bent with an O–S–O angle of about 120° (experimental value near 119°).
Is SO₂ polar or nonpolar?
SO₂ is polar. The S–O bonds are polar, and the bent shape prevents the bond dipoles from canceling, so the molecule has a net dipole moment.