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Carbonic acid (H₂CO₃) Lewis structure

24 valence electrons, trigonal planar

C O O O H H
H₂CO₃ as the Lewis Structure Generator draws it: lone pairs as dots.

Build H₂CO₃ step by step in the Lewis Structure Generator

What is the Lewis structure of H₂CO₃?

Carbonic acid, H₂CO₃, has 24 valence electrons. Carbon is the central atom, joined by one C=O double bond, two C−O single bonds and two O−H single bonds. It carries two lone pairs on each O of an O−H and two on the double-bonded O. Every formal charge is zero. C is AX₃: trigonal planar, and the molecule is polar.

Valence electrons in H₂CO₃

Element Group Valence electrons Atoms Electrons
Hydrogen 1 1 2 2
Carbon 14 4 1 4
Oxygen 16 6 3 18
Total 24

In one line: 2 × 1 + 4 + 3 × 6 = 24. Each atom brings the valence electrons its group gives it.

How to draw the H₂CO₃ Lewis structure

  1. Count the valence electrons. Valence electrons: each H 1, C 4 and each O 6. 2 × 1 + 4 + 3 × 6 = 24.
  2. Choose the central atom. C goes in the middle: C is the least electronegative atom other than H, χ 2.55 against 3.44 for O. Each H goes on its own O, as in any acid written with H first.
  3. Join the atoms with single bonds. Five single bonds use 10 electrons, leaving 24 − 10 = 14.
  4. Complete the outer atoms. Each outer atom takes enough lone pairs to reach eight, H needing nothing more than its bond: 2 O(H) × 4 + O 6 = 14, leaving 14 − 14 = 0.
  5. Put the rest on the central atom. Nothing is left, so C gets no lone pair, and C now has 6 electrons round it.
  6. Make multiple bonds where needed. C has only 6 electrons, so one lone pair moves into the bonds, making a C=O bond, and C now has 8.

Lone pairs and formal charges in H₂CO₃

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

Minor resonance contributors to H₂CO₃

Ranked by the same rules, the next best arrangements are valid Lewis structures that count for less in the real molecule. One that can be drawn either way round is shown once.

C O O O H H + −
+1 on the O with one double bond and one single bond and −1 on the single-bonded O. Its formal charges add up to more: 2 against 0.

Shape and polarity of H₂CO₃

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

The O−H groups break the symmetry of the bonds round the center, as in HNO₃, H₂SO₄ and H₃PO₄, which are all polar.

C 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 H₂CO₃