3k. The covalent bonding in the carbon dioxide CO₂ molecule with extra notes on its properties and uses

Bonding diagrams for CO₂  O=C=O

Covalent bonding diagrams for CARBON DIOXIDE

A covalent molecule, molecular formula CO₂

One atom of carbon (2.4) combines with two atoms of oxygen (2.6) to form the compound carbon dioxide CO₂ (only the outer shell of carbon's electrons are shown).

The above left 'Venn diagram' style is the best representation of the electronic structure of the triatomic linear carbon dioxide molecule.

Carbon is four electrons short of a full outer shell (8 electrons) and oxygen is two electrons short of a full outer shell (8 electrons), so one carbon atom shares its four outer electrons with two outer electrons from each of the oxygen atoms, so all three atoms now have a full outer shell of 8 electrons in the formation of two double bonds (O=C=O).

Electronically, carbon (2.4) becomes like neon (2.8) and oxygen (2.6), also becomes like neon (2.8), so the hydrogen and carbon atoms effectively have full outer shells in forming the covalent bonds when the atoms share their outer electrons.

The diagram above left is the best style for carbon dioxide, clearly showing the sharing of the pairs of electrons for the two double C=O covalent bonds in the carbon dioxide molecule (in a sort of Venn diagram style).

(Lewis diagram of carbon dioxide) simplified 'dot and cross' electronic diagram for the covalently bonded carbon dioxide molecule.

The right diagram is the full 'dot and cross' electronic diagram for the covalent bonding in the carbon dioxide molecule.

The electronic dot and cross Lewis diagrams for covalent bonding in carbon dioxide

The molecule can be shown as  (displayed formula) with two carbon = oxygen double covalent bonds.

(Advanced note: CO₂ has a linear shape, and the O=C=O bond angle is 180^o.

The valencies of C and O are 4 and 2 respectively.

The carbon dioxide molecule is held together by the strong C=O carbon–oxygen double covalent bonds by sharing electrons.

Comments

The 'melting point' of solid carbon dioxide is -78 ^oC, but it changes immediately into a gas (sublimes). This low state change temperature indicates the intermolecular bonds (intermolecular forces) are weak between the molecules, contrasting with the very strong C=O bonds in the molecule - sometimes referred to as the intramolecular bonding.

The weak intermolecular bonds means the particle kinetic energies do not need to be so high to overcome the intermolecular bonds in melting or boiling, hence the low melting/boiling point (note that carbon dioxide actually sublimes at -78^oC at normal pressure).

Carbon dioxide doesn't have a boiling point at normal atmospheric pressure, because the solid starts to sublime at -78^oC ('dry ice'). Solid carbon dioxide gives a smoke effect when added to water - what you see on the 'stage' is clouds of condensed water vapour in the denser carbon dioxide gas because of the low temperature created by the CO₂ bubbles up out of the water.

The strong double bonds between the atoms in the carbon dioxide molecule make it chemically quite stable and this stability allows it to accumulate in the atmosphere and absorb infrared radiation contributing to the greenhouse effect.

See Greenhouse effect and global warming from fossil fuel burning

Compressed carbon dioxide is used in fire extinguishers.

Carbon dioxide is used in freeze drying food

Carbon dioxide can be liquefied at low temperatures and high pressure. The coffee beans are soaked in water so they swell up. In carbon dioxide decaffeination of coffee, the coffee is placed, at low temperature, in a container called an extraction chamber. Liquid carbon dioxide is then forced at very high pressure into the extraction chamber, dissolving out the caffeine and water from the coffee. The liquefied carbon dioxide acts as a solvent. When the coffee beans are free of caffeine, the carbon dioxide is removed and the beans dried and roasted to give the commercial coffee product.

Carbon dioxide gas is produced when acids react with carbonates or hydrogencarbonates

See Acid reactions with metals/oxides/hydroxides/carbonates/ammonia, neutralisation to give salts

Extra structured, exam-board-aligned summary notes on carbon dioxide (CO₂)

tailored for GCSE/IGCSE Chemistry students across WJEC, CCEA, CIE, AQA, Edexcel, OCR Gateway, and OCR 21st Century specifications.

Some GCSE level chemistry practise exam questions on the bonding, structure and properties of carbon dioxide

Bonding and Structure of Carbon Dioxide (CO₂)

Type of Bonding in CO₂

• Covalent bonding between carbon and oxygen atoms.
• Carbon shares two pairs of electrons with each oxygen atom → double covalent bonds.

Electron Configuration of CO₂

• Carbon: 4 outer electrons → needs 4 more.
• Oxygen: 6 outer electrons → needs 2 more.
• Dot-and-cross diagrams show two double bonds and no lone pairs on carbon.

Molecular Geometry of the carbon dioxide molecule

• Linear shape.
• Bond angle = 180° due to no lone pairs on central atom and equal repulsion.
• Symmetrical molecule → non-polar overall, despite polar bonds.

Intermolecular Forces in carbon dioxide

• Weak van der Waals forces between molecules.
• This is why ammonia has a very low melting and boiling point.
• No hydrogen bonding.

Physical Properties of carbon dioxide

Uses of Carbon Dioxide - industrial and Everyday Uses

Chemistry, Laboratory and Analytical Uses

• Test for CO₂: limewater turns cloudy due to formation of calcium carbonate.
• Used in acid-base reactions and produced in thermal decomposition experiments.
• Carbon dioxide in fizzy drinks creates carbonic acid, which adds a slightly sour-tart taste and enhances the overall flavour profile. The CO₂ bubbles also carry aromas to your nose, intensifying the drink's taste, and have a palate-cleansing effect that balances sweetness.

Common Misconceptions about carbon dioxide

Exam Tips about questions involving carbon dioxide

• Use dot-and-cross diagrams to show double bonds clearly.
• Explain linear shape using electron pair repulsion theory (advanced level).
• Link structure to properties (e.g. gas at room temp due to weak forces).
• Know the limewater test and its limitations.
• Understand CO₂’s role in photosynthesis, combustion, and climate change.

Learning objectives for the bonding in a carbon dioxide molecule

Recognise the position of carbon and oxygen in the periodic table - both non-metals.

Know that when two non-metallic elements combine, the chemical bond is most likely to be covalent.

Know that for simple molecules like carbon dioxide, the atoms try to attain an electronic structure like a noble gas by sharing their out electrons.

Know and be able to describe and explain with a diagram the formation of the covalent bonds in the carbon dioxide molecule by electron sharing (between the two positive nuclei of the carbon and oxygen atoms).

Know what is meant by, and be able to describe and explain the formation of a double bond e.g. in the carbon dioxide molecule.

Know that carbon dioxide has a very low melting point and boiling point because the forces between the molecules are very weak - weak intermolecular forces/bonding.