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ScienceQuest

Sulfur dioxide (SO₂) Lewis structure

18 valence electrons, bent

S O O
SO₂ as the Lewis Structure Generator draws it: lone pairs as dots.

Build SO₂ step by step in the Lewis Structure Generator

What is the Lewis structure of SO₂?

Sulfur dioxide, SO₂, has 18 valence electrons. Sulfur is the central atom, joined by two S=O double bonds. It carries one lone pair on the central S and two on each O. Every formal charge is zero. S is AX₂E₁: bent, with an ideal bond angle of 120°, and the molecule is polar.

Valence electrons in SO₂

Element Group Valence electrons Atoms Electrons
Sulfur 16 6 1 6
Oxygen 16 6 2 12
Total 18

In one line: 6 + 2 × 6 = 18. Each atom brings the valence electrons its group gives it.

How to draw the SO₂ Lewis structure

  1. Count the valence electrons. Valence electrons: S 6 and each O 6. 6 + 2 × 6 = 18.
  2. Choose the central atom. S goes in the middle: S is the least electronegative atom other than H, χ 2.58 against 3.44 for O. Checked the other way, O in the middle would need an O−O bond, and electronegative atoms of one element avoid bonding to each other when they can.
  3. Join the atoms with single bonds. Two single bonds use 4 electrons, leaving 18 − 4 = 14.
  4. Complete the outer atoms. Each outer atom takes enough lone pairs to reach eight: 2 O × 6 = 12, leaving 14 − 12 = 2.
  5. Put the rest on the central atom. The 2 left go on the central atom as one lone pair on S, and S now has 6 electrons round it.
  6. Make multiple bonds where needed. S has only 6 electrons, so two lone pairs move into the bonds, making two S=O bonds, and S now has 10.

Lone pairs and formal charges in SO₂

Atom Number Bonds Lone pairs Electrons round it Formal charge, V − N − B/2
S (central) 1 two double bonds 1 10 6 − 2 − 4 = 0
O 2 one double bond 2 8 6 − 4 − 2 = 0

V is the atom’s valence electrons, N its nonbonding electrons and B its bonding electrons. The formal charges add up to 0, the molecule’s overall charge.

SO₂ with an octet on every atom

The structure above keeps the formal charges as small as they go and puts 10 electrons round S. Keeping every atom to eight instead, the octets-first convention, gives two equivalent forms. Each has one S=O double bond and one S−O single bond, with formal charges of +1 on the central S and −1 on the single-bonded O. Courses differ on which of the two they expect.

S O O + −
Octets first, form 1
S O O + −
Octets first, form 2

Draw SO₂ with octets first in the Lewis Structure Generator

Shape and polarity of SO₂

Central atom S
VSEPR class AX₂E₁
Electron geometry trigonal planar
Molecular geometry bent
Ideal bond angle 120°
Measured bond angle 119°
Polarity, an estimate polar

The bent shape leaves the bond dipoles unbalanced.

S has two σ bonds and one lone pair, three electron domains in all, so its electron geometry is trigonal planar. Counting only the bonded atoms, the molecular geometry is bent.

The VSEPR Molecular Geometry viewer turns the AX₂E₁ shape in 3D, lone pairs included.

The measured angle is the accepted experimental value the VSEPR viewer quotes for this molecule.

Work with SO₂