carbon monoxide lewis structure
The carbon monoxide lewis structure that gives both atoms complete octets is a multiple-bond structure written as :C≡O:, with one lone pair on carbon and one lone pair on oxygen; the corresponding formal charges are carbon negative and oxygen positive.
Valence-electron accounting and octet completion
- Total valence electrons: carbon contributes 4 and oxygen contributes 6, for a total of \(4 + 6 = 10\) valence electrons.
- Octet target: two second-period atoms require \(8 + 8 = 16\) electrons around them; shared (bonding) electrons count toward both octets.
- Octet-satisfying arrangement: three shared pairs between C and O (a triple bond) plus one lone pair on each atom uses \(6 + 2 + 2 = 10\) electrons and completes both octets.
The electron bookkeeping above is the key constraint for CO: 10 total valence electrons is too few for a double-bond structure with two lone pairs on each atom, but it is exactly enough for a triple bond with one lone pair on each atom.
Formal charges in the preferred Lewis structure
Formal charge is evaluated by \[ \text{FC} = V - \left(N + \frac{B}{2}\right), \] where \(V\) is valence electrons for the free atom, \(N\) is nonbonding electrons assigned to the atom, and \(B\) is bonding electrons shared in bonds.
| Atom | \(V\) | \(N\) | \(B\) | Formal charge |
|---|---|---|---|---|
| Carbon (C) | 4 | 2 | 6 | \(\text{FC}_\text{C} = 4 - \left(2 + \frac{6}{2}\right) = 4 - 5 = -1\) |
| Oxygen (O) | 6 | 2 | 6 | \(\text{FC}_\text{O} = 6 - \left(2 + \frac{6}{2}\right) = 6 - 5 = +1\) |
The formal-charge pattern \( \text{C}^{-}\) and \( \text{O}^{+}\) can feel counterintuitive because oxygen is more electronegative than carbon; carbon monoxide is a classic example where formal charge is a bookkeeping tool and does not directly equal measured partial charge distribution.
Bonding interpretation
- Bond order in the Lewis picture: three shared electron pairs correspond to a triple bond (one \(\sigma\) bond and two \(\pi\) bonds in a localized-bond model).
- Lone pairs: one lone pair resides on carbon and one lone pair resides on oxygen in the octet-satisfying structure.
- Net charge: the molecule is neutral because \((-1) + (+1) = 0\).
Common pitfalls
- Electron total mismatch: CO has 10 valence electrons, so a drawing that uses 12 or more electrons is not consistent with the neutral molecule.
- Octet violation in second-row atoms: carbon and oxygen follow the octet rule; expanded-octet drawings for CO are not appropriate.
- Formal charge omitted: the octet-satisfying Lewis structure includes nonzero formal charges; leaving charges off can hide the electron-allocation result.
carbon monoxide lewis structure conventions summarize CO as :C≡O: with one lone pair on each atom and formal charges C⁻ and O⁺, satisfying octets with 10 total valence electrons.