Covalent bonding
A covalent bond forms when two non-metal atoms share a pair of electrons. Each atom achieves a stable noble-gas outer shell (usually 8, or 2 for hydrogen and helium).
Simple molecules
Common GCSE examples:
| Molecule | Formula | Bonds |
|---|---|---|
| Hydrogen | H_2 | 1 single |
| Hydrogen chloride | HCl | 1 single |
| Water | H_2O | 2 single |
| Methane | CH_4 | 4 single |
| Ammonia | NH_3 | 3 single |
| Oxygen | O_2 | 1 double |
| Nitrogen | N_2 | 1 triple |
| Carbon dioxide | CO_2 | 2 double |
A double bond shares two pairs (4 electrons); a triple bond shares three pairs.
Dot-and-cross diagrams
Show outer-shell electrons only. Use one symbol (dot) for one atom and another (cross) for the other so you can see what each atom contributes. Only the shared pairs sit in the overlap of the two atomic shells.
Properties of simple molecular substances
- Low melting and boiling points — weak intermolecular forces between molecules need only a little energy to overcome (the strong covalent bonds inside the molecule are NOT broken on melting or boiling).
- Do not conduct electricity — molecules are neutral; no free ions or electrons.
- Many are gases or liquids at room temperature.
WJEC exam tip
When asked why methane has a low boiling point, do not say "the covalent bonds are weak". They are strong. The answer is: weak intermolecular forces between separate methane molecules need only a little energy to overcome. Always specify "between molecules".
AI-generated · claude-opus-4-7 · v3-wjec-combined-science-leaves