Covalent bonding
Covalent bonds
A is a between two atoms. Covalent bonds form between atoms. By sharing, each atom gets a full outer shell.
Covalent bonds are very strong. Substances with covalent bonds can be:
- , such as H₂O, Cl₂ and CH₄
- very large molecules, such as
- , such as diamond and silicon dioxide
A single line in a structural formula, such as H–Cl, stands for one covalent bond.
Key terms
Dot and cross diagrams for molecules
Draw each atom's as a circle. Circles overlap where atoms bond, with each shared pair in the overlap: a dot from one atom and a cross from the other. Unshared electrons stay in their own circle.
| Molecule | Formula | Bonds |
|---|---|---|
| Hydrogen | H₂ | 1 single (H–H) |
| Chlorine | Cl₂ | 1 single (Cl–Cl) |
| Hydrogen chloride | HCl | 1 single (H–Cl) |
| Water | H₂O | 2 single (O–H) |
| Ammonia | NH₃ | 3 single (N–H) |
| Methane | CH₄ | 4 single (C–H) |
| Oxygen | O₂ | 1 double (O=O): 2 shared pairs |
| Nitrogen | N₂ | 1 triple (N≡N): 3 shared pairs |
Check: every atom except hydrogen should end with 8 outer electrons; hydrogen ends with 2.
Key terms
Properties of small molecules
Substances made of are usually gases or liquids at room temperature, with .
That's because there are only between the molecules. When the substance melts or boils, it is these weak forces that are overcome — not the strong covalent bonds inside each molecule. The intermolecular forces get stronger as the molecules get bigger, so larger molecules have higher boiling points.
Small molecules do not conduct electricity, because the molecules have no overall charge and there are no free electrons.