The CH2O Lewis structure corresponds to formaldehyde (also written as H2CO). The dominant structure features a carbonyl group with a C=O double bond, two C–H single bonds, and two lone pairs on oxygen, satisfying the octet rule for carbon and oxygen in a second-period molecule.
CH2O Lewis structure: \( \mathrm{H_2C=O} \) with two lone pairs on O and zero formal charge on all atoms.
Valence-electron total and electron accounting
The CH2O Lewis structure begins with the total number of valence electrons contributed by each atom in the neutral molecule: carbon contributes 4, oxygen contributes 6, and two hydrogens contribute \(2 \times 1\). The total is \(4 + 6 + 2 = 12\) valence electrons, arranged as bonding pairs and lone pairs.
| Atom | Valence electrons per atom | Number of atoms | Total contribution |
|---|---|---|---|
| C | 4 | 1 | 4 |
| O | 6 | 1 | 6 |
| H | 1 | 2 | 2 |
| Total | 12 |
Bonding pattern and octet satisfaction
In the CH2O Lewis structure, carbon forms three electron domains: two single bonds to hydrogen and one double bond to oxygen. The carbon atom reaches an octet by sharing four electron pairs total (two in the C=O double bond and one in each C–H bond). Oxygen reaches an octet by participating in the double bond (two shared pairs) and retaining two lone pairs (two nonbonding pairs). Each hydrogen reaches a duet through its single bond to carbon.
Formal charge consistency
Formal charge values confirm the stability of the dominant Lewis structure. Using \( \mathrm{FC = (valence\ e^-) - (nonbonding\ e^-) - \tfrac{1}{2}(bonding\ e^-)} \), carbon in \( \mathrm{H_2C=O} \) has FC = 0, oxygen has FC = 0, and each hydrogen has FC = 0. The absence of charge separation matches the typical neutral representation of formaldehyde.
Resonance and the carbonyl group
The carbonyl group admits a secondary charge-separated resonance form, smaller in contribution than the neutral form:
The resonance pair highlights polarization of the C=O bond, with electron density drawn toward oxygen. This polarization is central to the reactivity of carbonyl compounds in general chemistry and organic chemistry contexts.
Molecular geometry and bonding implications
Around carbon in CH2O, three electron domains (two C–H bonds and one C=O region) correspond to a trigonal planar electron-domain geometry and a planar molecular framework with bond angles near \(120^\circ\). The C=O bond is polar due to electronegativity differences, and the oxygen lone pairs contribute to the electron-rich region on the oxygen end of the molecule.
Common pitfalls
- Incorrect bond order: a single C–O bond with three lone pairs on oxygen leaves carbon short of an octet unless charge separation is introduced.
- Lone-pair count on oxygen: two lone pairs in the neutral dominant structure, not one and not three.
- Atom placement: carbon as the central atom with hydrogen terminal positions and oxygen terminal position in a carbonyl.