Non-metal atoms join together by sharing pairs of electrons. A shared pair of electrons is a covalent bond. Small groups of atoms joined in this way form simple molecules.
Forming covalent bonds
By sharing electrons, each atom gains a full, stable outer shell. Hydrogen achieves 2 outer electrons, while most other atoms achieve 8 (an octet). Atoms can share one, two or three pairs of electrons, giving single, double or triple bonds. The covalent bonds hold the atoms tightly together inside each molecule. Dot-and-cross diagrams are used to show which electrons come from each atom in the shared pairs.
| Molecule | Bonding |
|---|---|
| H₂, Cl₂ | one single bond |
| H₂O | two single bonds |
| CH₄ | four single bonds |
| O₂ | one double bond |
| N₂ | one triple bond |
Key idea
The covalent bonds within a molecule are strong, but the forces between molecules are weak. Only these weak intermolecular forces are overcome on melting or boiling.
Properties of simple molecular substances
Because only the weak forces between molecules need to be broken when they change state, simple molecular substances have low melting and boiling points and many are gases or liquids at room temperature. They do not conduct electricity, because the molecules are neutral and there are no free electrons or ions to carry charge. Most are insoluble in water but dissolve in organic solvents such as ethanol. Note that although the substance melts easily, the strong covalent bonds inside the molecules are not broken — only the weak forces between whole molecules are overcome.
Remember
- Simple molecular substances have low melting and boiling points.
- They do not conduct electricity (no free electrons or ions).
- Covalent bonding occurs between non-metal atoms.