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ScienceQuest

Formaldehyde (CH₂O) Lewis structure

12 valence electrons, trigonal planar

C H O H
CH₂O as the Lewis Structure Generator draws it: lone pairs as dots.

Build CH₂O step by step in the Lewis Structure Generator

What is the Lewis structure of CH₂O?

Formaldehyde, CH₂O, has 12 valence electrons. Carbon is the central atom, joined by one C=O double bond and two C−H single bonds. It carries two lone pairs on the O. Every formal charge is zero. C is AX₃: trigonal planar, with an ideal bond angle of 120°, and the molecule is polar.

Valence electrons in CH₂O

Element Group Valence electrons Atoms Electrons
Carbon 14 4 1 4
Hydrogen 1 1 2 2
Oxygen 16 6 1 6
Total 12

In one line: 4 + 2 × 1 + 6 = 12. Each atom brings the valence electrons its group gives it.

How to draw the CH₂O Lewis structure

  1. Count the valence electrons. Valence electrons: C 4, each H 1 and O 6. 4 + 2 × 1 + 6 = 12.
  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. Checked the other way, with O in the middle the formal charges add up to 4, against 0 with C.
  3. Join the atoms with single bonds. Three single bonds use 6 electrons, leaving 12 − 6 = 6.
  4. Complete the outer atoms. The outer O takes 6 electrons as lone pairs to reach eight, leaving 6 − 6 = 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 CH₂O

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
H 2 one single bond 0 2 1 − 0 − 1 = 0
O 1 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.

Shape and polarity of CH₂O

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

Bonds of different polarity do not cancel in this shape.

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 CH₂O