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1.7Principles of Chemistry

Covalent Bonding — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 7 parts Lesson 15 of 25 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Lesson summary

Covalent Bonding: Molecules, Dot-and-Cross Diagrams and Giant Structures — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Explain covalent bonding, draw dot-and-cross diagrams for common molecules and compare simple molecular and giant covalent substances.Covalent bonding occurs between …

  • intermolecular forces
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Double bond

Part 1 of 5Covalent bonds

Covalent bonding occurs between non-metal atoms, which share pairs of outer electrons so that each atom has a full outer shell.

Covalent bond
A covalent bond is a shared pair of electrons, held between the two nuclei by electrostatic attraction.
The bonds are strong.
The number of bonds an atom makes equals the number of electrons it needs to fill its outer shell: hydrogen 1, oxygen 2, nitrogen 3, carbon 4, chlorine 1.

Covalent bonding explains why non-metal compounds exist as molecules.

Covalent bonds
The shared electrons are attracted to both nuclei at once, and this attraction holds the atoms together.
Atoms in a molecule also arrange themselves in particular shapes; for example the four bonds in methane point to the corners of a tetrahedron, water is bent and carbon dioxide is linear, though you only need the dot-and-cross diagrams at this level.

Part 2 of 5Dot-and-cross diagrams of molecules

MoleculeBonding
Hydrogen H2One shared pair (single bond), each H has 2 electrons
Chlorine Cl2One shared pair; each Cl has three lone pairs
Water H2OTwo single bonds; oxygen has two lone pairs
Ammonia NH3Three single bonds; nitrogen has one lone pair
Methane CH4Four single bonds
Hydrogen chloride HClOne shared pair
Oxygen O2Double bond (two shared pairs)
Nitrogen N2Triple bond (three shared pairs)
Carbon dioxide CO2Two double bonds, O=C=O

In a dot-and-cross diagram only outer electrons are shown; use dots for one atom and crosses for the other so you can see where electrons come from.

Part 3 of 5Simple molecular substances

Small molecules have strong covalent bonds inside them but only weak intermolecular forces between them.

Simple molecular substances
Therefore they have low melting and boiling points (little energy needed to overcome the weak forces), they are often gases or liquids at room temperature,
  • and they do not conduct electricity because there are no free charged particles.
When a simple molecular substance melts or boils, only the weak intermolecular forces are overcome, not the covalent bonds.

Comparing the properties of covalent substances in a table helps in exams: simple molecular substances (water, methane, iodine) are gases, liquids or low melting point solids, do not conduct electricity and are often soluble in organic solvents rather than water; giant covalent substances (diamond, silicon dioxide) are hard solids with very high melting points that are insoluble in water. Iodine is a solid that sublimes easily because its molecules are held by weak forces.

Part 4 of 5Giant covalent structures

In giant covalent substances, millions of atoms are joined by strong covalent bonds in a continuous network.

1
Diamond (each carbon bonded to four others) is very hard with a very high melting point and does not conduct.
2
Graphite has layers of carbon atoms each bonded to three others, with delocalised electrons that conduct,
  • and weak forces between layers so it is soft and slippery.
3
Silicon dioxide is hard with a high melting point.
4
Fullerenes such as C60 are carbon atoms joined in hollow cages and are simple molecules.
5
Covalent bonds between atoms are very strong; a high melting point means many strong bonds must be broken.

When asked to explain why a substance conducts or does not conduct, name the particle that moves: metals and graphite have delocalised electrons, ionic liquids have mobile ions, and covalent molecules have neither. A frequent error is to say that covalent substances do not conduct because their bonds are strong; the correct reason is that they have no free electrons or ions.

Part 5 of 5Worked examples

  1. Draw the dot-and-cross diagram for ammonia. [2]2 marks

  2. Explain why methane has a low boiling point. [2]2 marks

  3. Why can graphite conduct electricity but diamond cannot? [2]2 marks

  4. How many shared pairs are in a nitrogen molecule? [1]1 mark

Key terms

TermMeaning
Covalent bondShared pair of electrons between two atoms.
Lone pairPair of outer electrons not used in bonding.
Intermolecular forceWeak force between molecules.
Giant covalentNetwork of atoms joined by covalent bonds.
Double bondTwo shared pairs of electrons.

Avoid these mistakes

1
Saying covalent bonds break when water boils.
2
Forgetting lone pairs in diagrams.
3
Saying all covalent substances have low melting points — giant covalent ones do not.
4
Thinking simple molecules conduct electricity.

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