The key idea
Covalent bonds form when non-metal atoms share pairs of electrons.Simple molecular substances have low melting points; giant covalent structures have very high melting points.
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Formation of covalent bonds
A covalent bond forms when two non-metal atoms each contribute one electron to a shared pair.Both atoms are held together by the electrostatic attraction between their positive nuclei and the shared pair of electrons.The number of covalent bonds an atom forms equals the number of additional electrons it needs to achieve a full outer shell.
Simple molecular structures
Substances like water (H₂O), methane (CH₄), ammonia (NH₃) and chlorine (Cl₂) consist of small molecules held together by strong covalent bonds within the molecule.However, the forces between separate molecules (intermolecular forces) are weak.This gives these substances low melting and boiling points.They generally do not conduct electricity because they have no charged particles.
Giant covalent structures
Some non-metal elements and compounds form giant covalent (macromolecular) structures, where every atom is bonded to several others by strong covalent bonds throughout the whole structure.Diamond, graphite and silicon dioxide (SiO₂) are examples.These have very high melting points because vast amounts of energy are needed to break the many strong covalent bonds.
Diamond and graphite
Diamond has a tetrahedral giant covalent structure in which each carbon forms four strong covalent bonds; it is very hard and does not conduct electricity.Graphite has a layered structure in which each carbon forms three covalent bonds, leaving one delocalised electron per atom; these electrons allow graphite to conduct electricity.The layers in graphite can slide over each other, making it a useful lubricant.
Definitions to learn
Covalent bond
A shared pair of electrons between two non-metal atoms.
Simple molecular substance
A substance made of small molecules with strong covalent bonds inside and weak intermolecular forces between molecules.
Giant covalent structure
A structure in which all atoms are joined by strong covalent bonds throughout, giving high melting points.
Intermolecular force
A weak force of attraction between separate molecules in a simple molecular substance.
Delocalised electron
An electron not associated with a single atom but free to move through a structure (as in graphite or metals).
Dot-and-cross diagram
A diagram showing the outer electrons of atoms as dots or crosses to illustrate covalent bonding.
Draw a dot-and-cross diagram to show the bonding in a water molecule (H₂O).
Oxygen has 6 outer electrons; each hydrogen has 1.
Oxygen needs 2 more electrons to complete its outer shell.
Each hydrogen atom shares its electron with the oxygen, forming 2 covalent bonds.
Draw O with 2 bonding pairs and 2 lone pairs; each H with one bonding pair.
Each O–H bond is a shared pair of electrons (one from O, one from H).
When asked about properties of covalent substances, first identify whether the structure is simple molecular or giant covalent — they have very different properties.
Do not say covalent compounds have high melting points as a rule.Simple molecular substances have low melting points; giant covalent structures have high melting points.
How to score full marks
- 1For melting point questions, identify the structure FIRST: simple molecular = low; giant covalent = very high.
- 2When explaining graphite's conductivity, always say 'delocalised electrons that are free to move' — mentioning electrons alone is not enough.
- 3Covalent bonds are strong; intermolecular forces are weak — examiners deduct marks if you confuse these when explaining melting points.
Try these yourself
Open each answer only after you have explained the full chemical process.
1Define a covalent bond.[1 mark]
- 1.Use the words 'shared' and 'electrons'.
2State the number of covalent bonds in a molecule of methane (CH₄).[1 mark]
- 1.Count the bonds between C and each H.
3Explain why simple molecular substances such as iodine have low melting points.[3 marks]
- 1.Distinguish intermolecular forces from covalent bonds.
4Explain why diamond has a very high melting point.[3 marks]
- 1.Describe the giant covalent structure.
5State one difference in structure between diamond and graphite.[2 marks]
- 1.Compare the number of bonds per carbon.
6Explain why graphite conducts electricity.[3 marks]
- 1.Consider the delocalised electrons.
7Compare the boiling points of methane (CH₄) and silicon dioxide (SiO₂) and explain the difference in terms of structure and bonding.[4 marks]
- 1.Identify the structure type of each compound.
8Explain why most simple covalent compounds do not conduct electricity.[3 marks]
- 1.Consider the particles and their charge.