Chemistry · Bonding, Structure and Properties of Matter
Ionic Compounds
Spec 5.2.1.3📙 Higher
Interactive · change the state, test the lattice
Theory · read, then commit
When an ionic compound forms, the ions don't stay in pairs — they build a giant ionic lattice: a regular 3-D arrangement of alternating positive and negative ions.
🧊Regular 3-D lattice
Positive and negative ions alternate in all directions; in NaCl each ion is surrounded by 6 of the opposite charge.
🔗Strong forces all directions
Billions of ions are held by strong electrostatic attraction throughout the crystal.
Checkpoint · quick check
Describe the structure of a giant ionic lattice.
🌡️High melting & boiling point
A lot of energy is needed to overcome the strong forces (NaCl 801 °C, MgO 2852 °C).
🚫No conduction when solid
The ions are locked in fixed positions, so no charge can move.
⚡Conducts molten or dissolved
Now the ions are free to move, so they can carry charge.
💥Hard but brittle
A knock shifts the layers so like charges meet and repel — the crystal shatters.
Checkpoint · quick check
Explain why solid sodium chloride does not conduct electricity, but molten sodium chloride does.
⚠️ Common Mistake
Students often say that because ionic compounds contain charged ions they should conduct electricity even when solid. They do not: in a solid the ions are locked in fixed positions in the lattice and cannot move. Only when the compound is melted or dissolved are the ions free to move — and it is moving charged particles that carry a current.
🧂Example — sodium chloride & solubility
NaCl: Na⁺ and Cl⁻ in a 1:1 ratio throughout the lattice.
Many ionic compounds dissolve — water molecules surround the ions and pull them from the lattice.
Some are insoluble and precipitate: Ag⁺(aq) + Cl⁻(aq) → AgCl (white solid).
Retrieval · Ionic Compound Property — Explain Why
Test yourself · 10 questions, exam order
⭐Examiner Tip
For conduction marks you must say the ions are free to move. Solid ionic compounds don't conduct because the ions are locked in the lattice; only when molten or dissolved can the ions move and carry charge. "It contains charged particles" on its own scores nothing.
0 of 10 answered
Warm-up · 2 questions
Explain why ionic compounds have high melting points.
Ionic compounds are hard but brittle. Explain why they shatter when hit.
Exam standard · 4 questions
Explain why many ionic compounds dissolve in water and the solution then conducts electricity.
Magnesium oxide has a much higher melting point than sodium chloride. Explain why.
Explain, in terms of structure and bonding, why ionic compounds are hard and brittle whereas metals are malleable.
A white solid melts at about 800°C and conducts electricity when molten but not when solid. Deduce its structure and bonding, and justify your answer using its properties.
Stretch · 4 questions
Explain why ionic compounds do not conduct electricity in the solid state, even though they are made of charged ions.
Suggest why not all ionic compounds are soluble in water.
Explain why the formula NaCl represents the simplest ratio of ions rather than a molecule made of one sodium atom and one chlorine atom.
Predict, with reasons, whether calcium oxide (CaO) or potassium chloride (KCl) has the higher melting point.
Key note · cover it, say it, check it
Revision card · 5 stepsIonic Compounds
01Giant ionic lattice: regular 3D arrangement of alternating + and − ions held by strong electrostatic forces in all directions.
02High MP/BP (strong forces).
03Brittle (layers shift → ion repulsion).
04Solid: no conduction (ions fixed).
05Molten/dissolved: conducts (ions free to move).
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