IGCSE Chemistry 0620: Double Covalent Bonding in Carbon Dioxide

A concert production crew uses blocks of dry ice, solid carbon dioxide ( $\text{CO}_2$ ), to create a low-lying fog effect on stage. As the blocks warm up, they change directly from a solid into carbon dioxide gas.

(a) Describe, in terms of electrons, the covalent bonding between the carbon atom and each oxygen atom in a molecule of carbon dioxide, including the number of electron pairs shared in each bond. [4]

(b) State the type of structure formed by carbon dioxide molecules, and explain why carbon dioxide can change from a solid to a gas at a much lower temperature than a giant covalent structure such as silicon dioxide changes from a solid to a liquid. [3]

Part (a): Double covalent bonding in carbon dioxide

Carbon is in Group IV, so a carbon atom has 4 electrons in its outer shell. Oxygen is in Group VI, so each oxygen atom has 6 electrons in its outer shell.

In carbon dioxide, carbon forms a double covalent bond with each of the two oxygen atoms. A double bond means two shared pairs of electrons are formed between the two atoms, rather than the single shared pair of an ordinary covalent bond:

$\text{O} = \text{C} = \text{O}$

For each double bond, carbon contributes 2 of its own electrons (one to each of the two shared pairs) and the oxygen atom contributes 2 of its own electrons (one to each of the two shared pairs). Across its two double bonds, carbon uses all 4 of its outer electrons. Counting the shared pairs towards carbon’s outer shell, carbon ends up with a share of 8 electrons (4 shared pairs $\times$ 2 electrons).

Each oxygen atom contributes 2 of its 6 outer electrons to its double bond with carbon, and keeps its remaining 4 electrons as 2 lone (non-bonding) pairs. Counting its shared pairs and lone pairs together, each oxygen atom also ends up with a full outer shell of 8 electrons (4 from the 2 shared pairs $+$ 4 from the 2 lone pairs).

Part (b): Structure of carbon dioxide and comparison with silicon dioxide

Carbon dioxide has a simple molecular structure: it exists as small, separate $\text{CO}_2$ molecules, with only weak forces of attraction acting between neighbouring molecules. Changing carbon dioxide from a solid to a gas only requires overcoming these weak intermolecular forces, the strong C=O covalent bonds within each molecule remain intact, so this happens at a relatively low temperature.

Silicon dioxide, however, has a giant covalent structure, in which every silicon and oxygen atom is joined to its neighbours by strong covalent bonds extending continuously throughout the whole structure, with no separate molecules. Changing its state means breaking a huge number of these strong covalent bonds throughout the lattice, which requires far more energy, giving silicon dioxide a much higher temperature for a change of state than carbon dioxide.

Part (c): Electrical conductivity of carbon dioxide

Carbon dioxide gas does not conduct electricity. Every one of carbon’s and oxygen’s outer-shell electrons is held within a fixed covalent bond inside each molecule, so there are no free ions and no delocalised electrons available to move through the structure and carry a charge.

Final answers

(a) Double covalent bond between carbon and each oxygen (2 shared pairs per bond); carbon ends with 8 outer electrons (from 4 shared pairs total), each oxygen ends with 8 (4 from its 2 shared pairs $+$ 4 from its 2 lone pairs).
(b) Simple molecular structure; only weak forces between molecules are overcome (low temperature for change of state), unlike silicon dioxide’s giant covalent structure, where many strong covalent bonds must be broken (much higher temperature).
(c) Carbon dioxide does not conduct electricity, no free ions or delocalised electrons.