Carbon monoxide Lewis Structure

Carbon Monoxide Lewis Structure: CO Lewis Structure, Triple Bond, sp Hybridization & Polarity

Carbon monoxide Key Facts

The carbon monoxide Lewis structure is C≡O, a triple bond (one σ and two π bonds) with one lone pair on each atom; the molecule is linear, carbon is sp-hybridized, the C≡O axis is 180°, and although formally charge-separated in the best resonance form, CO has only a small dipole and is best described as only weakly polar.

Figure 1: Lewis Structure of Carbon monoxide (CO)

Chemical Formula	CO
CAS Number	630-08-0
Total Valence Electrons	10
Lewis Structure	C≡O with one lone pair on each atom
Electron-Domain Geometry	Linear
Molecular Geometry	Linear (diatomic)
Hybridization Model	sp (both atoms)
C≡O Bond Angle	180°
Polarity	Weakly polar (small dipole)

PubChem and NIST identify carbon monoxide as CO with CAS 630-08-0; PubChem lists a molecular weight of approximately 28.010 g/mol. View CAS 630-08-0 dictionary details on Guidechem →

Applications of Carbon monoxide (CAS 630-08-0)

Carbon monoxide is a deceptively simple diatomic that is central to industrial reduction and C1 chemistry. Its dominant use is as a reducing agent in metallurgy: in blast furnaces and direct-reduction processes, CO reduces metal oxides (notably iron ore) to the corresponding metals.

CO is also a key component of syngas (with H₂), which is catalytically converted to methanol, to hydrocarbons via Fischer–Tropsch, and to acetic acid and related oxygenates through carbonylation. It serves as a fuel gas, a chemical intermediate for phosgene and isocyanates (under strict control), and a ligand in organometallic chemistry because of its strong π-acceptor character.

Metallurgical Reduction Primary reducing agent for iron ore and other metal oxides in steel and alloy production.

Syngas & Fischer–Tropsch Component of syngas converted to methanol, hydrocarbons and fuels.

Carbonylation (Acetic Acid) Monsanto/Rhône-Poulenc-type carbonylation of methanol to acetic acid and derivatives.

Procurement Note Carbon monoxide is highly toxic, colorless and odorless; procurement and use require certified gas-handling infrastructure, continuous monitoring and strict safety protocols. Supply is typically as cylinders or bulk, often co-produced with H₂ as syngas. Always reference CAS 630-08-0 and confirm purity and impurity (CO₂, H₂, N₂) limits for the intended process.

Step 1 - Valence Electrons & Central Atom

Counting valence electrons for CO: carbon is in Group 14 and contributes 4 valence electrons; oxygen is in Group 16 and contributes 6 valence electrons.

Valence-electron count

Total valence electrons = 4 from C + 6 from O = 10 electrons.

Step 2 - Lewis Structure & Electron Distribution

A single C–O bond uses 2 electrons, leaving 8. Placing three lone pairs on each atom leaves carbon with only 2 bonding electrons (no octet). To satisfy both octets, the bond order must increase.

The best Lewis representation is a triple bond C≡O with one lone pair remaining on each atom. The triple bond uses 6 electrons, leaving 4 electrons as one lone pair on carbon and one on oxygen. In the dominant resonance form carbon carries a formal negative charge and oxygen a formal positive charge (:C≡O:), the reverse of the electronegativity expectation.

C ≡ O

One lone pair on C and one lone pair on O

:C≡O: (10 valence electrons, both atoms octet)

Octet & Formal Charge in CO With C≡O and one lone pair on each atom, carbon has 2 (lone pair) + 6 (triple bond) = 8 electrons and oxygen likewise has 8 — both achieve octets. Formal charges: carbon 4 − 2 − ½×6 = −1; oxygen 6 − 2 − ½×6 = +1. This surprising :C≡O: charge distribution is confirmed by the molecule's small dipole, which points from O toward C (opposite to the electronegativity trend).

Step 3 - VSEPR Theory & Geometry

CO is diatomic, so its geometry is trivially linear — two atoms define a straight line and the bond axis is 180°. There are no lone-pair-vs-bonding-angle complications as in triatomic molecules.

From a VSEPR standpoint, each atom has one bonding domain plus one lone pair; the bonding axis is linear by definition. The interesting structural story in CO is not the angle but the triple bond and its electron distribution.

Repulsion hierarchy

For a diatomic molecule there is a single bonding axis; the lone pairs on C and O lie along that axis on opposite sides of the bond. Repulsion between the lone pairs and the bonding pair simply reinforces the linear arrangement.

Hybridization & Formal Charge

Both carbon and oxygen in CO are described as sp-hybridized. One sp hybrid on each atom forms the σ component of the triple bond; the remaining two p orbitals on each atom form the two perpendicular π components. The second sp hybrid on each atom holds the lone pair.

This sp picture naturally yields a linear molecule with a short, strong triple bond (the strongest stable bond between two atoms).

Formal-charge check

Formal charge = valence − nonbonding − ½ × bonding. Carbon in :C≡O:: 4 − 2 − ½×6 = −1. Oxygen: 6 − 2 − ½×6 = +1. The dominant resonance form is therefore :C≡O:⁺ with a negative charge on carbon — a counterintuitive but experimentally supported result.

Modeling Note The :C≡O: formal-charge pattern reflects real electron density: carbon in CO is the donor end (the lone pair binds to metals), and the dipole is small and points O→C. Molecular-orbital theory explains this through σ donation and π back-donation; the sp model captures the linear geometry and triple bond but should not be read as implying a large C⁻–O⁺ permanent separation.

Is Carbon monoxide Polar or Nonpolar?

Carbon monoxide is only weakly polar. The C≡O bond is polar, but the dominant Lewis form :C≡O: places a formal negative charge on carbon and positive on oxygen, so the electrostatic and bond-dipole contributions partly oppose each other.

The resulting molecular dipole is small (about 0.11 D) and points from oxygen toward carbon — the opposite direction to what simple electronegativity would predict. For most practical purposes CO is treated as effectively nonpolar, though it binds strongly to metal centers via its carbon lone pair.

Expert Quote — Structural Chemistry Perspective

"CO is the classic exception that proves the rule. Its dipole is tiny and points the ‘wrong’ way because the dominant Lewis structure :C≡O: puts the negative charge on carbon. The molecule's chemistry — its strong binding to metals through carbon — follows directly from that electron distribution."

Interpretation consistent with MO theory and coordination-chemistry descriptions of carbon monoxide.

This unusual polarity and the carbon lone pair make CO an excellent π-acceptor ligand in organometallic chemistry (metal carbonyls), while its reducing power drives metallurgical and syngas processes. The same triple bond that makes CO reactive also makes it a potent and dangerous toxin by binding hemoglobin.

Structural Comparison Table

Species	Formula	Bond Order	Electron Domains	Geometry	Polarity
Carbon monoxide	CO	3 (triple)	2 per atom	Linear	Weakly polar
Carbon dioxide	CO₂	2 (each C=O)	2 (central C)	Linear	Nonpolar
Nitrogen	N₂	3 (triple)	2 per atom	Linear	Nonpolar
Nitric oxide	NO	2.5	2 per atom	Linear	Polar
Dinitrogen monoxide	N₂O	mixed	2 (central)	Linear	Polar

Practice question Draw the CO Lewis structure and calculate formal charges on C and O. Explain why the dipole points from O to C despite oxygen being more electronegative, and relate this to CO's ability to bind metals through carbon.

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FAQs

What is the Lewis structure of CO?

:C≡O: with one lone pair on each atom. A triple bond satisfies both octets and gives formal charges of −1 on C and +1 on O.

Is CO polar or nonpolar?

CO is only weakly polar. The dipole is small (~0.11 D) and points O→C because the dominant form :C≡O: puts negative charge on carbon.

What is the hybridization in CO?

Both C and O are sp-hybridized. One sp forms the σ bond, two p orbitals form the two π bonds, and the second sp holds the lone pair.

How many valence electrons does CO have?

10 valence electrons. Four from carbon plus six from oxygen.

What is the CAS number of carbon monoxide?

The CAS number is 630-08-0. PubChem (CID 281) and NIST identify this with CO.

References

PubChem, Carbon Monoxide, CID 281. Molecular formula, CAS number and molecular weight (≈28.010 g/mol).
NIST Chemistry WebBook, SRD 69, Carbon Monoxide. Formula, CAS Registry Number, thermochemical and spectroscopic data.
IUPAC Gold Book, "formal charge" and "dipole moment." Definitions underpinning the CO charge/dipole discussion.
Miessler, G. & Tarr, D., Inorganic Chemistry. MO treatment of CO, the :C≡O: dipole and metal-carbonyl bonding.
Twigg, M. V. (Ed.), Catalyst Handbook. Industrial use of CO in syngas, Fischer–Tropsch and carbonylation.

Technical scope: Lewis structures, VSEPR and hybridization are model-based descriptions for chemical communication; real bonding in CO is best described by molecular orbitals (σ donation + π back-donation). CAS 630-08-0 identifies the substance. CO is highly toxic and must be handled under controlled gas-safety conditions.

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Carbon monoxide Lewis Structure