2.5Atoms, Elements and Compounds

Simple Molecules and Covalent Bonds — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

5 min read 11 parts Lesson 5 of 7

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Lesson summary

Explain covalent bonding, draw dot-and-cross diagrams for simple molecules and explain the properties of simple molecular substances.

Part 1 of 9How a covalent bond forms

Covalent bond
- a pair of electrons shared between two atoms.
Who bonds
- only non-metal atoms form covalent bonds with each other.
- Aim
  - each atom gets a full outer shell, giving a noble gas configuration.
- Molecule
  - two or more atoms joined by covalent bonds.

Electron symbols
- electrons from one atom are dots and from the other are crosses.
Overlap
- the shells of the bonded atoms overlap and the shared electrons sit in the overlap.
- Origin
  - the diagram shows which atom each electron came from.
- Full shells
  - check every atom has a full shell, remembering that the first shell holds only 2.
Ionic or covalent
- a metal plus a non-metal is ionic, so draw separate ions in brackets with charges.
Non-metals only
- covalent, so draw overlapping shells with shared pairs.

Single bond
- two adjacent atoms share one pair of electrons.
Hydrogen H₂ and chlorine Cl₂
- each atom shares one pair.
- Hydrogen chloride HCl
  - one shared pair between H and Cl.
- Water H₂O
  - oxygen shares one pair with each of two hydrogens.
- Methane CH₄
  - carbon shares one pair with each of four hydrogens.
- Ammonia NH₃
  - nitrogen shares one pair with each of three hydrogens and keeps one non-bonding pair.

Double bond
- two atoms share two pairs of electrons.
Ethene
- the two carbon atoms share two pairs.
- Carbon dioxide
  - carbon shares two pairs with each oxygen atom.
- Methanol
  - contains single bonds only, including C–O and O–H.
Triple bond
- two atoms share three pairs of electrons.
Nitrogen N₂
- each nitrogen shares three pairs to complete its outer shell.

Low melting and boiling points
- so they are usually liquids or gases at room temperature.
Reason
- only weak intermolecular forces act between the molecules.
- Compared with bonds
  - these forces are about one tenth as strong as covalent bonds.
- Trend
  - as molecules get bigger there are more electrons, the intermolecular forces grow and the melting and boiling points rise.
Poor electrical conductivity
- there are no free ions or electrons to carry charge.
Insulators
- most covalent substances do not conduct in the solid state.
- Examples
  - plastic coating on wires, rubber and wood.

Feature	Covalent bond	Intermolecular force
Where it acts	Between atoms inside a molecule	Between neighbouring molecules
Strength	Strong	Weak
Broken when	Reacting chemically	Melting or boiling a simple molecular substance

Water
- the O–H bonds are covalent, while the attractions between water molecules are intermolecular forces.

Compound contains	Bonding	Diagram shows
Metal and non-metal	Ionic	Separate ions in square brackets with charges
Non-metals only	Covalent	Overlapping shells with shared pairs of electrons

Check before drawing
- identify whether each element is a metal or a non-metal.
Overall charge
- ionic diagrams must total zero charge.
- Full shells
  - covalent diagrams must give each atom a full outer shell.

Pattern
- the number of bonds an atom makes equals the number of electrons it needs to complete its outer shell.
Carbon
- four bonds.
- Nitrogen
  - three bonds.
- Oxygen
  - two bonds.
- Hydrogen and halogens
  - one bond.

Describe how a covalent bond forms between two chlorine atoms. [2]2 marks
1. Each chlorine atom has 7 outer electrons and needs one more.
2. The atoms share one pair of electrons, so each has a full outer shell of 8.
Draw and explain the dot-and-cross diagram for nitrogen N₂. [3]3 marks
1. Each nitrogen atom has 5 outer electrons.
2. Three pairs are shared between the atoms, forming a triple bond.
3. Each atom then has 8 outer electrons, including one non-bonding pair on each.
Explain why methane has a low boiling point. [2]2 marks
1. Methane is a simple molecule with only weak intermolecular forces between molecules.
2. Little energy is needed to overcome them, and the covalent bonds do not break.
Explain why solid iodine, a simple molecular substance, does not conduct electricity. [1]1 mark
1. It has no free ions or electrons to carry charge.

Term	Meaning
Covalent bond	A shared pair of electrons between two atoms.
Molecule	Two or more atoms joined by covalent bonds.
Double bond	Two shared pairs of electrons between two atoms.
Triple bond	Three shared pairs of electrons between two atoms.
Intermolecular force	A weak attraction between neighbouring molecules.
Insulator	A substance that does not conduct electricity.

Saying covalent bonds break when a simple molecular substance melts — only weak intermolecular forces are overcome.
Drawing ions with brackets for a covalent molecule — covalent molecules show overlapping shells.
Forgetting that hydrogen needs only two electrons in its shell.
Confusing a double bond with two single bonds — it is two shared pairs between the same two atoms.

Last updated: 19 October 2026