Covalent Bonding and Simple Molecular Substances — AQA GCSE Chemistry
Covalent bonding occurs between non-metal atoms by the sharing of electrons.
How covalent bonds form
When non-metal atoms combine, they share pairs of electrons. Each shared pair is a covalent bond. By sharing, each atom gains a full outer shell.
Examples of simple molecules: H₂, Cl₂, O₂, H₂O, CO₂, CH₄, NH₃.
Representing molecules
You can show covalent bonds using:
- dot-and-cross diagrams (showing shared electron pairs),
- displayed formulae (a line for each bond, e.g. H–H), and
- ball-and-stick models.
Properties of simple molecular substances
- Low melting and boiling points — the covalent bonds within molecules are strong, but the intermolecular forces between molecules are weak and easily overcome. Many are gases or liquids at room temperature.
- As molecules get larger, the intermolecular forces increase, so melting and boiling points rise.
- They do not conduct electricity — there are no free electrons or ions.
Common misconception
When a simple molecular substance melts or boils, it is the weak intermolecular forces that are broken — not the strong covalent bonds.
Exam tips
- Covalent bonding = non-metals sharing electron pairs.
- Low melting/boiling points are due to weak intermolecular forces.
- They don't conduct (no free electrons or ions).
- Be able to draw dot-and-cross diagrams for simple molecules.