Simple molecules and covalent bonds

O Level / IGCSE Chemistry · Atoms, elements and compounds. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

A covalent bond forms when two atoms share a pair of electrons. It happens between non-metal atoms. By sharing, each atom gets the electronic configuration of a noble gas: 2 outer electrons for hydrogen and 8 for most other atoms.

One shared pair is a single bond, two shared pairs make a double bond and three make a triple bond. Outer electrons that are not shared are called lone pairs. A dot-and-cross diagram shows which atom each electron came from.

The bonds inside a molecule are strong, but the intermolecular forces between one molecule and the next are weak. This is why simple molecular substances melt and boil at low temperatures and do not conduct electricity.

Rules to remember

Common mistake

When a simple molecular substance melts or boils, the covalent bonds do not break. Only the weak forces between the molecules are overcome, and the molecules stay whole.

Worked example

How many pairs of electrons are shared in one molecule of carbon dioxide, CO2, and how many lone pairs are on each oxygen atom? (C is 2,4; O is 2,6)

  1. Carbon has 4 outer electrons and needs 4 more. Each oxygen has 6 outer electrons and needs 2 more.
  2. Carbon shares two pairs with each oxygen atom, so there are two double bonds: 2 × 2 = 4 shared pairs.
  3. Each oxygen uses 2 of its 6 outer electrons in bonding, leaving 6 − 2 = 4 electrons, which is 2 lone pairs.
  4. Check: carbon has 4 shared pairs = 8 electrons; each oxygen has 4 shared + 4 unshared = 8 electrons.

Answer: 4 shared pairs (two double bonds) and 2 lone pairs on each oxygen atom.

Practice questions

Try each one, then open the answer.

1. Iodine crystals turn into a purple vapour on gentle warming. What is overcome when this happens?

  1. A
    the covalent bonds between the atoms in each iodine molecule
  2. B
    metallic bonds between iodine atoms
  3. C
    the weak forces between iodine molecules
  4. D
    ionic bonds between iodine ions
Show answer

Answer: C. Iodine is a simple molecular substance. Gentle warming only separates the I2 molecules; the strong covalent bond inside each molecule is not broken.

2. Which substance does not conduct electricity either as a solid or when molten?

  1. A
    sulfur
  2. B
    copper
  3. C
    graphite
  4. D
    lead(II) bromide
Show answer

Answer: A. Sulfur is a simple molecular substance with no free electrons or ions in any state. Copper and graphite have free electrons; molten lead(II) bromide has mobile ions.

3. Which molecule contains a double covalent bond?

  1. A
    CO2
  2. B
    NH3
  3. C
    CH4
  4. D
    HCl
Show answer

Answer: A. In CO2 each oxygen shares two pairs of electrons with carbon: O=C=O. The other molecules contain only single bonds.

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