Ionic lattices and multicharged ions
Explain ionic properties with a repeating lattice and balance charges beyond a simple 1:1 example.
Exam scope and assessment notes
Extended only (Supplement) · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Explain cation and anion formation.
- Read the charges on common ions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many chloride ions balance one Mg²⁺ ion?
What you will learn
- Describe a giant ionic lattice and connect it to melting and conduction.
- Draw ionic formation with more than one transferred electron.
A lattice is an extended pattern
An ionic crystal has a repeating three-dimensional arrangement of positive and negative ions. Attractions act in many directions, rather than inside isolated molecules. High melting and boiling points reflect the large energy needed to overcome enough strong electrostatic attractions to separate or rearrange the ions.
A solid contains charged ions, but they cannot move freely through its fixed lattice, so it does not conduct. When molten, the ions can move and carry charge. In an aqueous solution of a soluble ionic compound, separated mobile ions can also carry charge. Electrons moving between metal ions are not the mechanism in an ionic melt.
Balance the total charge
Magnesium loses two electrons to form Mg²⁺. Two chlorine atoms each gain one electron to form two Cl⁻ ions. The ratio is therefore one magnesium ion to two chloride ions, giving MgCl₂. In a diagram, show the Mg²⁺ ion and both separately bracketed chloride ions; each chloride's complete outer shell contains one electron originally from magnesium.
For magnesium oxide, Mg²⁺ and O²⁻ balance in a 1:1 ratio. Oxygen gains two electrons to reach a full outer shell, with both transferred electrons distinguished in its diagram. Formula ratios follow charge balance; they do not always equal the number of electrons transferred from one atom.
Pause and explain
What carries current through molten magnesium oxide?
Worked example
Explain why solid magnesium chloride does not conduct but molten magnesium chloride does.
Show the worked solution
- The solid has Mg²⁺ and Cl⁻ ions in a fixed lattice.
- Those ions cannot travel through the solid to carry current.
- When molten, mobile ions carry charge through the liquid.
Answer Conduction requires mobile ions; melting frees the lattice ions to move.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe an outer-shell dot-and-cross diagram for MgCl₂ formed from Mg (2,8,2) and Cl (2,8,7).
Give me a hint
Each chlorine receives one of magnesium's two outer electrons.
Compare my reasoning
- Draw one bracketed Mg²⁺ and two bracketed Cl⁻ ions.
- Each chloride has eight outer electrons with one originally from magnesium.
- Check that +2 and two −1 charges sum to zero.
One Mg²⁺, two Cl⁻; each chloride receives one electron.
Look for these in your work
- The diagram represents both anions.
- Full outer shells and total charge are checked.
Explain the high melting point of magnesium oxide and give the electron transfer forming its ions.
Give me a hint
Use strong attractions throughout the lattice and a two-electron transfer.
Compare my reasoning
- Magnesium loses two electrons and oxygen gains two, making Mg²⁺ and O²⁻.
- These ions form a repeating lattice with strong electrostatic attractions.
- Much energy is needed to overcome the attractions for melting.
Two-electron transfer makes Mg²⁺ and O²⁻; strong lattice attractions require much energy to overcome.
Look for these in your work
- Both ion charges follow the transfer.
- Melting is explained using attractions, rather than strong covalent molecules.
A solid has high melting point, conducts when molten and when dissolved, but not when solid. Explain why an ionic lattice fits and whether the evidence alone names the compound.
Give me a hint
Match each observation to a structural feature; distinguish inference from identification.
Compare my reasoning
- High melting suggests strong attractions in an extended structure.
- State-dependent conduction fits fixed ions becoming mobile when molten or dissolved.
- Several ionic compounds share these properties, so the exact substance cannot be named from this evidence alone.
An ionic lattice fits; the evidence does not identify one specific compound.
Look for these in your work
- Melting and conduction have particle explanations.
- The limit of the identification is acknowledged.
Common mistakes
- Using mobile electrons to explain an ionic melt.
- Giving MgCl instead of balancing one +2 ion with two −1 ions.
What can you explain now?
What ion ratio balances Al³⁺ with O²⁻?
Compare with the explanation
2 aluminium ions to 3 oxide ions
Two +3 ions give +6 and three −2 ions give −6, giving the neutral ratio in Al₂O₃.
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Make it stick
Explain all three conduction states without using the phrase 'free electrons'. Then draw the charged particles needed for MgCl₂ and MgO.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.