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Sulfur trioxide (SO₃) Lewis structure

24 valence electrons, trigonal planar

S O O O
SO₃ as the Lewis Structure Generator draws it: lone pairs as dots.

Build SO₃ step by step in the Lewis Structure Generator

What is the Lewis structure of SO₃?

Sulfur trioxide, SO₃, has 24 valence electrons. Sulfur is the central atom, joined by three S=O double bonds. It carries two lone pairs on each O. Every formal charge is zero. S is AX₃: trigonal planar, with an ideal bond angle of 120°, and the molecule is nonpolar.

Valence electrons in SO₃

Element Group Valence electrons Atoms Electrons
Sulfur 16 6 1 6
Oxygen 16 6 3 18
Total 24

In one line: 6 + 3 × 6 = 24. Each atom brings the valence electrons its group gives it.

How to draw the SO₃ Lewis structure

  1. Count the valence electrons. Valence electrons: S 6 and each O 6. 6 + 3 × 6 = 24.
  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. Three single bonds use 6 electrons, leaving 24 − 6 = 18.
  4. Complete the outer atoms. Each outer atom takes enough lone pairs to reach eight: 3 O × 6 = 18, leaving 18 − 18 = 0.
  5. Put the rest on the central atom. Nothing is left, so S gets no lone pair, and S now has 6 electrons round it.
  6. Make multiple bonds where needed. S has only 6 electrons, so three lone pairs move into the bonds, making three S=O bonds, and S now has 12.

Lone pairs and formal charges in SO₃

Atom Number Bonds Lone pairs Electrons round it Formal charge, V − N − B/2
S (central) 1 three double bonds 0 12 6 − 0 − 6 = 0
O 3 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.

SO₃ with an octet on every atom

The structure above keeps the formal charges as small as they go and puts 12 electrons round S. Keeping every atom to eight instead, the octets-first convention, gives three equivalent forms. Each has one S=O double bond and two S−O single bonds, with formal charges of +2 on the central S and −1 on each single-bonded O. Courses differ on which of the two they expect.

S O O O 2+ − −
Octets first, form 1
S O O O 2+ − −
Octets first, form 2
S O O O 2+ − −
Octets first, form 3

Draw SO₃ with octets first in the Lewis Structure Generator

Shape and polarity of SO₃

Central atom S
VSEPR class AX₃
Electron geometry trigonal planar
Molecular geometry trigonal planar
Ideal bond angle 120°
Polarity, an estimate nonpolar

In a trigonal planar shape identical bonds cancel exactly.

S has three σ bonds and no lone pairs, three electron domains in all, so its electron geometry is trigonal planar. With no lone pairs, the molecular geometry is the same.

The VSEPR Molecular Geometry viewer turns the AX₃ shape in 3D, lone pairs included.

Work with SO₃