CID 280
CO2
Carbon dioxide
- C 1
- O 2
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- IUPAC name
- carbon dioxide
- Molar mass
- 44.0090(40) g/mol
- Atoms
- 3
- Shape
- linear
- Bond angle
- 180°
- Polarity
- nonpolar
- SMILES
- C(=O)=O
- InChIKey
- CURLTUGMZLYLDI-UHFFFAOYSA-N
- PubChem CID
- 280
The hiss of a soda can as it opens is carbon dioxide leaving the pressurised space above the drink. The same gas swells a loaf in the oven and bubbles off grape juice as it ferments into wine. It comes out of every flame that burns a carbon fuel, and animals release it as a product of their metabolism. Each molecule joins one carbon atom to two oxygen atoms, 44.009 g/mol in all.
The Hazardous Substances Data Bank puts the gas at 1.53 times the density of air, about 1.98 g/L at 0 °C, so it sinks and gathers in low places. Its phases are unusual too. The triple point of carbon dioxide lies well above 1 atm, which is why no liquid can form at ordinary pressure. A block of dry ice therefore never melts on a table: it sublimes, passing straight into gas at about −78 °C.
Structure and bonding
Carbon dioxide’s three atoms lie on one straight line, carbon in the middle and an oxygen at each end. Ball-and-stick draws each carbon–oxygen link as a double bond, and in every view the O–C–O angle is a full 180°. Turned end-on, the three atoms stack one behind another along the line of sight, which no bent molecule such as water can do. Space-filling merges the spheres into a short, symmetrical rod, and Stick leaves the two bonds as one straight line.
Carbon has two regions of electron density, one per double bond, and no lone pairs, and they repel least on opposite sides of it. Of three candidate electron-pair drawings, the drawn one wins because its formal charges, the charges atoms would carry if bonding electrons were shared evenly, are all zero.
Each C=O bond is polar, with a bond moment, a charge separation along it, pointing toward oxygen. The two moments point opposite ways and cancel, so the molecule is nonpolar. The viewer draws no charges, but it does show the straight shape that makes them cancel. In water some becomes carbonic acid, which the model does not show.
Where it turns up
Carbon dioxide is a normal part of air, according to NIOSH, the US National Institute for Occupational Safety and Health. Water carrying it dissolves limestone, because a soluble hydrogen carbonate forms, and that is how caves and sinkholes open up. Industry gathers much of its supply as a by-product of ammonia and hydrogen plants and of fermentation.
It is sold as a liquid in steel cylinders or as dry ice. In a fire extinguisher on a cool day of 18 °C, the liquid can be heard sloshing when the cylinder is shaken, and small canisters inflate life jackets. In the US, about 51 per cent of it goes to the food industry and about 18 per cent to carbonating drinks, on the Hazardous Substances Data Bank’s figures. Stored grain is kept free of insects under air holding about 60 per cent of the gas.
Why it matters
Plants draw carbon dioxide in through stomata, small regulated openings in the leaf, and use sunlight to build it and water into carbohydrates such as glucose. In human blood it travels three ways. About 76 per cent rides in the plasma, some as the dissolved gas and the rest as bicarbonate ion. The other 23 to 24 per cent binds to amino acids in haemoglobin, which carries it to the lungs to be traded for oxygen.
Carbon dioxide is a greenhouse gas. It absorbs heat radiating from the ground and re-releases it in all directions, some back towards the surface. Its three atoms are held loosely enough to vibrate when they take in heat. Nitrogen and oxygen, most of the air, do not absorb it. In 2025 the global average reached 425.62 parts per million, in the annual mean of the US National Oceanic and Atmospheric Administration (NOAA).
About a quarter of each year’s emissions dissolves in the ocean, where it reacts with water to form carbonic acid. Surface pH has fallen from 8.21 to 8.10 since the Industrial Revolution.
Properties
| Property | Value | Unit | Source |
|---|---|---|---|
| Molecular formula (Hill order) | CO2 | ||
| Molar mass | 44.0090(40) g/mol | g/mol | IUPAC/CIAAW 2024 |
| Carbon atoms | 1 | ||
| Oxygen atoms | 2 | ||
| Bonds | 2 | ||
| PubChem molecular weight | 44.009 g/mol | g/mol | PubChem |
Safety
In air, in drinks and in bread, carbon dioxide is relatively nontoxic and does not burn. The PubChem Laboratory Chemical Safety Summary lists three cautions. The heavy gas can build up in low or enclosed spaces and leave too little oxygen there; dry ice and the liquid cause frostbite; and cylinders hold it under pressure. The NIOSH card for workplace exposure also lists headache, dizziness and a raised heart rate among its symptoms.
Questions
Why is carbon dioxide a linear molecule?
Valence shell electron-pair repulsion theory (VSEPR) places the groups of electrons around a central atom as far apart as they can go. Carbon here has only two such groups, one for each double bond, and no lone pairs. Two groups get furthest apart on opposite sides of the atom, at 180°, so the oxygen atoms end up in a straight line with carbon between them, exactly as the model draws them.
Is CO2 polar or nonpolar?
Nonpolar, even though each of its bonds is polar. Water makes a useful contrast, because its two bonds are also polar. In water the lone pairs on oxygen bend the molecule, so its two bond moments, the charge separations along its bonds, do not cancel and the molecule is polar. Carbon dioxide has no lone pairs on its central atom, its two bond moments point in exactly opposite directions, and they add up to zero.
Why does soda go flat after the bottle is opened?
Drinks are carbonated under a high pressure of carbon dioxide, which pushes a lot of gas into solution, and in the sealed bottle the small space above the drink fills with pressurised gas. Opening the cap lets that gas escape, so the equilibrium shifts back and the dissolved carbon dioxide falls. The equilibrium settles again only slowly, which is why a freshly opened drink stays fizzy for a while. Joseph Priestley devised a way to make carbonated water in 1767.
Why does dry ice not melt into a liquid?
Liquid carbon dioxide can exist only at pressures well above 1 atm, because that is where its triple point lies. At ordinary pressure the solid turns straight to gas at −78.464 °C, a figure from the Hazardous Substances Data Bank. Heat is a limit as well as pressure: above its critical temperature, about 31 °C, no amount of pressure will turn the gas into a liquid.
Made of the same elements
- CO Carbon monoxide 2 atoms, 1 triple bond.
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.
- Why water is bent and carbon dioxide is straight: the VSEPR model Water's two bonds meet at 104.5°; carbon dioxide's lie in a straight line.
Sources
- PubChem CID 280, carbon dioxide
- IUPAC/CIAAW, Abridged Standard Atomic Weights 2024
- PubChem Laboratory Chemical Safety Summary for carbon dioxide (CID 280)
- NIOSH Pocket Guide to Chemical Hazards, Carbon dioxide
- OpenStax, Chemistry 2e, §7.6 Molecular Structure and Polarity
- OpenStax, Chemistry 2e, §7.4 Formal Charges and Resonance
- OpenStax, Chemistry 2e, §10.4 Phase Diagrams
- OpenStax, Chemistry 2e, §13.3 Shifting Equilibria: Le Châtelier's Principle
- OpenStax, Chemistry 2e, §18.6 Occurrence, Preparation, and Properties of Carbonates
- OpenStax, Chemistry 2e, §18.9 Occurrence, Preparation, and Compounds of Oxygen
- OpenStax, Biology 2e, §8.1 Overview of Photosynthesis
- OpenStax, Biology 2e, §7.5 Metabolism without Oxygen
- OpenStax, Anatomy and Physiology 2e, §18.3 Erythrocytes
- NOAA Global Monitoring Laboratory, Trends in Atmospheric Carbon Dioxide: global annual means
- NOAA Climate.gov, Climate Change: Atmospheric Carbon Dioxide
- UCAR Center for Science Education, The Greenhouse Effect
- NOAA Pacific Marine Environmental Laboratory, Ocean Acidification