Covalent bonding

Non-metal atoms bond by sharing pairs of electrons. Each shared pair is a covalent bond, and sharing lets every atom fill its outer shell. Most covalent substances are made of small molecules such as water, ammonia and methane: the bonds inside each molecule are strong, but the forces between the molecules are weak.

Key facts

  • A covalent bond is a shared pair of electrons between two non-metal atoms.
  • An atom forms as many bonds as the electrons it needs to fill its outer shell: hydrogen 1, oxygen 2, nitrogen 3 and carbon 4.
  • A double bond is two shared pairs (O=O, O=C=O); a triple bond is three (N≡N).
  • Pairs of outer electrons that are not used in bonds are called lone pairs.
  • Dot-and-cross diagrams show the outer electrons; displayed formulae show each bond as a line, like H–O–H.
  • Simple molecular substances have low melting and boiling points and do not conduct electricity.

Worked examples

Example 1

Draw a dot-and-cross diagram for water, H2O.

  1. Oxygen has 6 outer electrons and needs 2 more. Each hydrogen has 1 and needs 1 more.
  2. Oxygen shares one pair with each hydrogen: two single covalent bonds.
  3. Oxygen’s other 4 outer electrons make 2 lone pairs. Now oxygen has 8 outer electrons and each hydrogen has 2.

Answer: two shared pairs (O–H bonds) and two lone pairs on the oxygen

Example 2

Draw a dot-and-cross diagram for carbon dioxide, CO2.

  1. Carbon has 4 outer electrons and needs 4 more. Each oxygen has 6 and needs 2.
  2. Carbon shares two pairs with each oxygen: two double bonds. 2 × 2 = 4 shared pairs
  3. Each oxygen keeps 2 lone pairs, so every atom has 8 outer electrons.

Answer: O=C=O: two double bonds (4 shared pairs), and 2 lone pairs on each oxygen

Example 3

Methane, CH4, boils at −162°C even though its C–H bonds are strong. Why?

  1. Inside each methane molecule the four C–H covalent bonds are strong.
  2. When methane boils, the molecules move apart, but each molecule stays whole.
  3. Only the weak intermolecular forces are overcome, which needs little energy, so the boiling point is low.

Answer: boiling only overcomes the weak forces between molecules; the covalent bonds do not break