Launching your learning experience…
Detailed notes on Atoms, Elements and Compounds for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Ionic bonds form when electrons transfer from metals to non-metals, creating oppositely charged ions held by electrostatic attraction. Cambridge tests dot-and-cross diagrams, ionic properties, and formulae from ion charges.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Ionic bonds form when a metal atom transfers electrons to a non-metal atom, forming oppositely charged ions that attract each other.
Ionic bonding: the electrostatic force of attraction between oppositely charged ions formed by the transfer of electrons from a metal to a non-metal.
Formation of NaCl:
Formation of MgO:
Dot-and-cross diagrams for ionic compounds:
Deducing ionic formulae:
High melting points, conduct when dissolved/molten, brittle — all explained by the giant ionic lattice structure.
Giant ionic lattice: Ionic compounds form a regular 3D lattice of alternating positive and negative ions. Millions of electrostatic attractions in all directions.
Properties and explanations:
| Property | Explanation |
|---|---|
| High melting/boiling point | Many strong electrostatic forces between ions require a lot of energy to break |
| Conduct electricity when molten | Ions are FREE to move → carry charge |
| Conduct electricity when dissolved in water | Ions separate (dissociate) → free to move |
| Do NOT conduct when solid | Ions in fixed positions → cannot move to carry charge |
| Brittle | If layers are displaced, like charges align → strong repulsion → crystal shatters |
| Soluble in water (usually) | Water molecules attract and surround ions (hydration) |
| Insoluble in organic solvents | Organic solvents cannot break up the strong electrostatic forces |
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: Draw the dot-and-cross diagram for MgO or NaCl (3 marks — transfer shown, correct charges, correct configurations). 'Explain why sodium chloride has a high melting point' (2 marks — giant ionic lattice, many strong electrostatic forces). 'Explain why solid NaCl does not conduct electricity but aqueous NaCl does' (2 marks — solid: ions fixed; aqueous: ions free to move). Formulae from charges are tested in Paper 2 MCQ.
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 ions and ionic bonds, written exactly the way a tutor would explain them at the board.
Question
Draw a dot-and-cross diagram to show the ionic bonding in sodium chloride (NaCl). Indicate the charges on the ions formed.
Step-by-step solution
Step 1
Na has electronic configuration 2,8,1. It loses its single outer electron to form Na⁺ with configuration 2,8 (noble-gas configuration of Ne).
Step 2
Cl has electronic configuration 2,8,7. It gains one electron to form Cl⁻ with configuration 2,8,8 (noble-gas configuration of Ar).
Step 3
Show Na⁺ with a full outer shell of 8 electrons (using crosses from Na) and Cl⁻ with a full outer shell of 8 electrons (7 dots from Cl + 1 cross from Na). Label charges as Na⁺ and Cl⁻.
Answer
Na⁺ (2,8 configuration) and Cl⁻ (2,8,8 configuration) held together by electrostatic attraction; Na⁺ has a full diagram with [He] core + 8-electron outer shell (crosses); Cl⁻ has 8-electron outer shell (7 dots + 1 cross).
Examiner tip
Show electrons from Na as crosses and electrons from Cl as dots (or vice versa). Charge labels are essential. Square brackets around each ion with the charge outside are required.
Question
Explain why ionic compounds such as sodium chloride have high melting points.
Step-by-step solution
Step 1
In the solid state, ionic compounds form a giant ionic lattice — a regular three-dimensional arrangement of oppositely charged ions.
Step 2
Strong electrostatic forces of attraction act between each positive ion and surrounding negative ions in all directions throughout the lattice.
Step 3
A large amount of energy is needed to overcome these strong attractions and separate the ions; this requires a high temperature, giving a high melting point.
Answer
Strong electrostatic attraction between oppositely charged ions throughout the giant ionic lattice requires a large amount of energy to overcome; hence a high melting point.
Examiner tip
Key marking phrases: 'giant ionic lattice', 'strong electrostatic attraction', 'large amount of energy to overcome'.
Question
Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.
Step-by-step solution
Step 1
In solid NaCl, Na⁺ and Cl⁻ ions are held in fixed positions in the ionic lattice; they cannot move to carry electric charge.
Step 2
When NaCl is melted, the lattice breaks down; the ions become free to move through the melt.
Step 3
Free-moving ions can carry charge to the electrodes, so molten NaCl conducts electricity.
Answer
Solid: ions fixed in lattice, cannot move, no conduction. Molten: ions free to move, can carry charge, conducts electricity.
Examiner tip
The word 'free' is important in the mark scheme: 'free to move' or 'mobile ions'.
Question
Magnesium oxide (MgO) has a melting point of 2852°C, much higher than sodium chloride (801°C). Suggest why.
Step-by-step solution
Step 1
MgO contains Mg²⁺ and O²⁻ ions; NaCl contains Na⁺ and Cl⁻ ions.
Step 2
The charges on Mg²⁺ and O²⁻ (+2 and −2) are greater than those on Na⁺ and Cl⁻ (+1 and −1).
Step 3
Greater ionic charges → stronger electrostatic attraction between ions → more energy required to separate them → higher melting point.
Answer
Mg²⁺/O²⁻ have higher charges (+2, −2) than Na⁺/Cl⁻ (+1, −1); stronger electrostatic attraction; more energy to overcome → higher melting point.
Definitions to memorise and the exact keywords mark schemes credit for ions and ionic bonds answers — sharpened from recent examiner reports for the 2026 0654 sitting.
The strong electrostatic attraction between oppositely charged ions formed by the transfer of one or more electrons from a metal atom to a non-metal atom.
A positively charged ion formed when an atom loses one or more electrons (typically a metal).
A negatively charged ion formed when an atom gains one or more electrons (typically a non-metal).
A regular, three-dimensional arrangement of alternating positive and negative ions held together by strong electrostatic attractions in all directions.
The force of attraction between oppositely charged particles (e.g. positive and negative ions); the basis of ionic bonding.
The traps other students keep falling into on ions and ionic bonds questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2022
Why it happens
Students confuse ionic bonding (electron transfer) with covalent bonding (electron sharing).
How to avoid it
Ionic = transfer; covalent = share. Use 'transfer' in ionic questions and 'sharing' in covalent questions.
Why it happens
Students know ionic compounds have ions but forget they must be free to move.
How to avoid it
Solid: ions fixed in lattice — no conduction. Molten or dissolved: ions free to move — conduction.
0654 Examiner Report 2023
Why it happens
Students draw the electron configurations correctly but omit the charge labels.
How to avoid it
Always put square brackets around each ion and write the charge (e.g. Na⁺) outside the bracket.
The things students keep getting wrong in this sub-topic, answered.