CID 281

Carbon monoxide

Ball-and-stick Stick Space-filling

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2 atoms, 1 triple bond.

IUPAC name: carbon monoxide
Molar mass: 28.0100(30) g/mol
Atoms: 2
Polarity: polar
SMILES: [C-]#[O+]
InChIKey: UGFAIRIUMAVXCW-UHFFFAOYSA-N
PubChem CID: 281

Made of

Structural formula of carbon monoxide. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Carbon that burns incompletely stops partway to carbon dioxide, and the stopping point is carbon monoxide. OpenStax writes the burning of carbon as two steps: carbon monoxide forms first, giving out 111 kJ, then reacts further to carbon dioxide, giving out another 283 kJ. Each molecule pairs one carbon atom with one oxygen atom, for a molar mass of 28.01 g/mol.

The gas boils at −191.5 °C and burns in air with a bright blue flame. It has no colour and no smell. The US Centers for Disease Control and Prevention (CDC) call it a poison that kills without warning, claiming hundreds of lives every year. Industry makes it on purpose all the same. For 2023, US companies reported an aggregated volume of 4 billion to under 5.5 billion pounds to the Environmental Protection Agency.

Structure and bonding

The viewer holds two atoms, a grey carbon and a red oxygen, and the readout under it says “2 atoms, 1 triple bond”. With Labels on, the spheres carry the symbols C and O. OpenStax gives this molecule, with the cyanide ion, as an example of a triple bond, three electron pairs shared by one pair of atoms. Ball-and-stick draws that link as a triple bond. Stick strips the balls away and leaves a short rod, and Space-filling fuses the two spheres into one small, stubby capsule.

There is no central atom here, so the VSEPR (valence shell electron pair repulsion) shape words do not apply, and a two-atom model has no bond angle to show. The SMILES string in the facts list, a one-line text code for the structure, marks carbon −1 and oxygen +1. These are formal charges, a bookkeeping count of electrons, not charges the atoms actually carry.

The molecule is still polar. With a single bond, that bond’s dipole moment, its separation of charge, sets the polarity of the whole molecule. OpenStax calls CO’s dipole small, smaller than hydrogen fluoride’s. The viewer draws no charges, so the polarity is invisible in every view.

Where it turns up

Almost any flame fed by a carbon fuel can release carbon monoxide. The CDC’s list runs from cars, trucks, small engines and lanterns to stoves, gas ranges, fireplaces, furnaces and grills, and it adds that charcoal of every colour gives it off. Indoors, a chimney blocked by debris lets it build up, and a small leak in a car’s exhaust system can fill the cabin.

Combustion is not the only route. HSDB (the Hazardous Substances Data Bank), quoted in PubChem, adds the reduction of carbon dioxide and the breakdown of organic compounds such as aldehydes. Haz-Map, also quoted there, lists smoking, steel production, welding, firefighting and mining among activities with a risk of exposure. In aluminium smelting, carbon monoxide forms at the anode beside oxygen and carbon dioxide. Car catalytic converters catalyse its conversion, with unburnt hydrocarbons, into carbon dioxide and water vapour.

Why it matters

With hydrogen, carbon monoxide is the raw material for a wide variety of chemicals. Steam passed over coke at 1000 °C gives water gas, a mixture of the two, and OpenStax notes that methanol can be made from the same pair. HSDB lists products of carbon monoxide itself, such as acetic acid, formic acid and phosgene; the mixture, called syngas, feeds many others.

In an iron blast furnace, carbon monoxide forms in the hotter bottom regions and rises to reduce iron oxides to the pure metal. OpenStax names it the reducing agent there, the substance that turns iron(III) into iron metal.

Its toxicity gives it a second kind of weight. It binds haemoglobin more tightly than oxygen does, forming carboxyhaemoglobin and leaving the blood short of oxygen, and it binds myoglobin in muscle too. The CDC counts more than 400 Americans dying each year from unintentional carbon monoxide poisoning not linked to fires. In the same span, over 14,000 are admitted to hospital.

Properties

Properties of carbon monoxide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 281.
Property	Value	Unit	Source
Molecular formula (Hill order)	CO
Molar mass	28.0100(30) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	1
Oxygen atoms	1
Bonds	1
PubChem molecular weight	28.01 g/mol	g/mol	PubChem

Safety

Carbon monoxide is a toxic, extremely flammable gas that the senses cannot detect. Under the Globally Harmonized System (GHS) its signal word is Danger. Its hazard statements include toxic if inhaled, may damage fertility or the unborn child, and damage to organs from prolonged or repeated exposure. The CDC names infants, older people and those with heart disease, anaemia or breathing problems as the most likely to fall ill. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide card for carbon monoxide are the authoritative sheets.

Questions

Why is carbon monoxide so dangerous if you cannot smell it?

The gas is colourless and odourless, and the Emergency Response Guidebook states plainly that the sense of smell will not detect it. Its most common symptoms, headache, dizziness, weakness, vomiting, chest pain and confusion, are often described as flu-like, the CDC says. Someone asleep or drunk can die before any symptom appears, and the CDC advises battery-powered or battery back-up detectors near every sleeping area.

What does the carbon monoxide 3D model show?

It shows two atoms, carbon and oxygen, held by one triple bond: three electron pairs shared between the same two atoms. OpenStax states that a triple bond is both shorter and stronger than a double bond linking the same two atoms. Its table of average values puts C≡O at 1.13 Å and 1080 kJ/mol, against 1.23 Å and 741 kJ/mol for C=O. A molecule of two atoms has no bond angle.

Is carbon monoxide polar?

Slightly. Its one bond decides the polarity, and OpenStax calls that bond's dipole small. Nitrogen, N₂, has almost the same mass, about 28 atomic mass units, so both feel similar London dispersion forces, the weak attractions every molecule feels. Carbon monoxide adds dipole–dipole attractions, the pull between the charged ends of neighbouring polar molecules, so it is expected to boil higher than nonpolar N₂.

What is the difference between carbon monoxide and carbon dioxide?

Carbon dioxide holds two oxygen atoms on each carbon, carbon monoxide one. Both come from burning fossil fuels, and carbon monoxide can be oxidised further to carbon dioxide, the second step when carbon burns. Most nonmetal oxides are acidic, but OpenStax names carbon monoxide among the exceptions. Car catalytic converters speed the conversion of carbon monoxide into carbon dioxide.

Made of the same elements

CO₂ Carbon dioxide 3 atoms, 2 double bonds, linear.

Appears in these articles

A polar bond is not a polar molecule: carbon dioxide against water Carbon dioxide's two bonds are polar, yet the molecule is not.