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Detailed notes on Atoms, Elements and Compounds for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Covalent bonds form when non-metal atoms share pairs of electrons. Cambridge tests dot-and-cross diagrams, simple molecular vs giant covalent structures, and how structure explains physical properties.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Covalent bonds form between non-metal atoms. Each bond is a shared pair of electrons, giving each atom a full outer shell.
Covalent bond: the electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms. Forms between non-metal atoms.
Key molecules and their dot-and-cross diagrams:
| Molecule | Formula | Bonds | Structure |
|---|---|---|---|
| Hydrogen | H₂ | 1 single bond | H–H |
| Water | H₂O | 2 single bonds | V-shaped |
| Ammonia | NH₃ | 3 single bonds | Pyramidal |
| Methane | CH₄ | 4 single bonds | Tetrahedral |
| Chlorine | Cl₂ | 1 single bond | Cl–Cl |
| Oxygen | O₂ | 1 double bond | O=O |
| Carbon dioxide | CO₂ | 2 double bonds | O=C=O (linear) |
| Hydrogen chloride | HCl | 1 single bond | H–Cl |
Drawing dot-and-cross diagrams:
Simple molecules have strong covalent bonds within the molecule but weak intermolecular forces between molecules — hence low melting/boiling points.
Simple molecular substances: H₂O, CO₂, NH₃, CH₄, HCl, O₂, Cl₂, N₂, iodine (I₂).
Properties:
| Property | Explanation |
|---|---|
| Low melting and boiling points | Weak intermolecular forces between molecules — easy to overcome |
| Often gases or liquids at room temperature | Weak IMF |
| Do NOT conduct electricity | No charged particles free to move (molecules are neutral) |
| Often soluble in organic solvents | Similar intermolecular forces (like dissolves like) |
| Some dissolve in water | If molecules can form hydrogen bonds or ionise (e.g. HCl → H⁺ + Cl⁻) |
IMPORTANT distinction: The COVALENT BONDS within the molecule are strong (not broken at MP or BP). The INTERMOLECULAR FORCES between molecules are weak and are broken at MP/BP.
Diamond, graphite, and silicon dioxide have millions of covalent bonds throughout — explaining their extremely high melting points.
Giant covalent (macromolecular) structures contain millions of covalently bonded atoms in a 3D network.
Diamond:
Graphite:
Silicon dioxide (SiO₂):
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: Draw dot-and-cross diagram for H₂O or CO₂ (2 marks). 'Explain why diamond has a high melting point' (2 marks — giant covalent, many strong covalent bonds). 'Explain why graphite conducts electricity but diamond does not' (3 marks — delocalised electrons in graphite, free to move; diamond has no free electrons). 'Explain why CO₂ is a gas at room temperature' (2 marks — simple molecular, weak intermolecular forces). These are among the most commonly tested chemistry topics.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C3); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on molecules and covalent bonds, written exactly the way a tutor would explain them at the board.
Question
Draw the dot-and-cross diagram of a water molecule (H₂O), showing all outer-shell electrons.
Step-by-step solution
Step 1
Oxygen has 6 outer electrons; each hydrogen has 1 outer electron.
Step 2
Oxygen shares one electron with each hydrogen atom, forming two single O–H covalent bonds.
Step 3
After sharing, oxygen has 4 bonding electrons (2 pairs) and 4 non-bonding electrons (2 lone pairs); each hydrogen has 2 electrons (full first shell).
Answer
H₂O: oxygen at centre with two O–H single bonds and two lone pairs; each H has a shared pair giving full first shell.
Question
Explain why methane (CH₄) has a low boiling point of −161°C.
Step-by-step solution
Step 1
Methane is a simple molecular substance: small CH₄ molecules with strong covalent bonds within each molecule.
Step 2
Between molecules, only weak van der Waals (London dispersion) forces act.
Step 3
Very little energy is needed to overcome these weak intermolecular forces, so methane has a very low boiling point.
Answer
Methane is a simple molecular substance with only weak intermolecular (van der Waals) forces between molecules; little energy is needed to overcome them, giving a low boiling point.
Examiner tip
Do NOT say 'weak covalent bonds' — the covalent bonds within the molecule are STRONG. It is the forces BETWEEN molecules that are weak.
Question
Compare the structures of diamond and graphite and explain the differences in their electrical conductivity.
Step-by-step solution
Step 1
Diamond: each carbon atom is covalently bonded to four others in a rigid, three-dimensional tetrahedral lattice. All outer electrons are involved in C–C bonds; there are no free/delocalised electrons.
Step 2
Because diamond has no free electrons, it is a non-conductor of electricity.
Step 3
Graphite: each carbon is bonded to three others in flat hexagonal layers. Each carbon atom has one unhybridised electron that is delocalised across the layers.
Step 4
These delocalised electrons are free to move through the layers, allowing graphite to conduct electricity.
Answer
Diamond: all electrons in C–C bonds, no free electrons → non-conductor. Graphite: one delocalised electron per C, free to move between layers → conductor.
Examiner tip
Use the words 'delocalised' and 'free to move'. These are mark-scheme key words.
Question
Nitrogen exists as N₂ molecules. Explain what is meant by a triple bond in N₂ and state why N₂ is very unreactive.
Step-by-step solution
Step 1
A triple bond consists of three shared pairs of electrons (6 electrons shared) between the two nitrogen atoms.
Step 2
The triple bond is very strong (bond energy ~944 kJ/mol), requiring a very large amount of energy to break.
Step 3
Because so much energy is needed to break the N≡N bond, nitrogen is very unreactive at room temperature.
Answer
Triple bond = three shared pairs of electrons; very strong bond; requires large energy input to break → N₂ is very unreactive.
Definitions to memorise and the exact keywords mark schemes credit for molecules and covalent bonds answers — sharpened from recent examiner reports for the 2026 0654 sitting.
A bond formed by the sharing of a pair of electrons between two non-metal atoms; each atom contributes one electron to the shared pair.
A structure in which molecules consist of a small, fixed number of atoms held together by strong covalent bonds; the molecules themselves are attracted by weak intermolecular forces, giving low melting/boiling points.
A structure in which a very large number of atoms are all linked by covalent bonds in a continuous three-dimensional (or layered) network; examples include diamond, graphite, and silicon dioxide.
A pair of electrons in the outer shell of an atom that is not involved in a covalent bond.
Electrons that are not associated with a single atom or bond but are spread (delocalised) over a larger region; responsible for electrical conductivity in graphite and metals.
Weak intermolecular attractions between all molecules, arising from temporary fluctuations in electron distribution; they increase with molecular size.
The traps other students keep falling into on molecules and covalent bonds questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2023
Why it happens
Students attribute the low melting/boiling point to weak covalent bonds rather than weak intermolecular forces.
How to avoid it
Covalent bonds within the molecule are STRONG. The intermolecular forces BETWEEN molecules are weak. Always specify 'intermolecular forces' not 'covalent bonds'.
Why it happens
Students know diamond is a carbon allotrope like graphite, which does conduct.
How to avoid it
Diamond: every electron is in a C–C bond, no free electrons, non-conductor. Graphite: one free/delocalised electron per carbon, conductor.
Why it happens
Students put all electrons of one atom in the bond circle without including the contribution of the other atom.
How to avoid it
A single bond = 2 electrons (1 from each atom). Double bond = 4 electrons (2 pairs). Count outer electrons carefully before drawing.
The things students keep getting wrong in this sub-topic, answered.