Covalent bonding and dot-and-cross diagramsEdexcel A-Level Chemistry: Revision notes

A covalent bond is the strong electrostatic attraction between two nuclei and the shared pair of electrons between them. It forms between non-metal atoms. Each atom contributes one electron to the pair and, in most simple molecules, ends up with a full outer shell.

A single bond is one shared pair, a double bond two shared pairs and a triple bond three shared pairs. Pairs of outer electrons that are not shared are lone pairs.

Draw only the outer shells. Use dots for one atom's electrons and crosses for the other's, with the shared pair in the overlap.

• Single bonds: H₂, Cl₂, CH₄ (4 shared pairs), H₂O (2 bonding pairs, 2 lone pairs on O), NH₃ (3 bonding pairs, 1 lone pair on N).
• Double bonds: O₂, CO₂ (two C=O bonds, 2 lone pairs on each O), C₂H₄.
• Triple bond: N₂ (3 shared pairs, 1 lone pair on each N), HCN (H–C≡N).

Check that each atom has a full outer shell (8 electrons, or 2 for hydrogen) and that the number of shared electrons matches the bonds. Some molecules have fewer than 8 around the central atom, e.g. BF₃ (6).

In a dative covalent bond both electrons in the shared pair come from the same atom. The donor atom has a lone pair; the acceptor has an empty orbital (it is electron deficient or is a positive ion such as H⁺).

Once formed, a dative bond is identical to any other covalent bond. In a dot-and-cross diagram the dative pair is shown in the same way, often with an arrow from donor to acceptor.

Examples: NH₄⁺ (N lone pair donated to H⁺), H₃O⁺ (O lone pair donated to H⁺), the NH₃→BF₃ adduct and CO (an oxygen lone pair donated to carbon).

In AlCl₃ aluminium forms three covalent bonds and has only six outer electrons, so it is electron deficient. A chlorine atom in a second molecule uses a lone pair to form a dative bond to the aluminium. Two AlCl₃ molecules join as the dimer Al₂Cl₆.

In Al₂Cl₆ there are eight Al–Cl bonds: four around each aluminium, two of them dative (one at each Al, from a bridging Cl). Every aluminium now has eight outer electrons. This dimer forms in the vapour at lower temperatures; at high temperatures it breaks into AlCl₃ molecules.

The more shared pairs between two atoms, the greater the electron density between the nuclei, the stronger the attraction, and the closer the nuclei are pulled. So for the same two elements:

• bond length decreases: single > double > triple
• bond strength (bond enthalpy) increases: single < double < triple.

Example: C–C (0.154 nm, 347 kJ mol⁻¹), C=C (0.134 nm, 612 kJ mol⁻¹), C≡C (0.120 nm, 838 kJ mol⁻¹). Shorter bonds are stronger because the nuclei are closer to the shared electrons.

A double bond is less than twice as strong as a single bond, so it is not simply the sum of single bonds.