Properties of Ionic Compounds
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge IGCSE Chemistry (0620)
High Melting and Boiling Points
Ionic compounds typically have high melting and boiling points (e.g. NaCl: 801°C, MgO: 2852°C).
- The giant lattice is held together by strong electrostatic forces of attraction between oppositely charged ions, acting throughout the whole structure
- A large amount of energy is needed to overcome these forces and separate the ions
- Ions with a greater charge (e.g. Mg²⁺ and O²⁻) attract each other more strongly than singly-charged ions (e.g. Na⁺ and Cl⁻), giving an even higher melting point
Worked Example: Comparing Melting Points
- Question: explain why magnesium oxide (melting point 2852°C) has a much higher melting point than sodium chloride (melting point 801°C). [2]
- Answer: Mg²⁺ and O²⁻ ions have a greater charge than Na⁺ and Cl⁻ ions [1]; this produces stronger electrostatic forces of attraction between the ions in MgO, so more energy is needed to separate them, giving a higher melting point [1]
Electrical Conductivity
Whether an ionic compound conducts electricity depends entirely on whether its ions are free to move.
| State | Are ions free to move? | Conducts electricity? |
|---|---|---|
| Solid | No - fixed in the lattice | No |
| Molten | Yes - lattice has broken down | Yes |
| Aqueous (dissolved) | Yes - ions separated in solution | Yes |
The same ionic compound conducts when molten but not when solid, because only in the molten state are the ions free to move and carry charge
- Solid ionic compounds do contain charged particles (ions) - but the ions are locked in fixed positions in the lattice and cannot move, so no current flows
- When molten or dissolved, the lattice breaks down and the ions become free to move towards the oppositely charged electrode, carrying the current
Common Mistakes
- ❌ Thinking solid ionic compounds don't conduct because they contain no charged particles
- ✓ solid ionic compounds do contain ions (charged particles) - they simply cannot move, since electrical conduction requires charged particles that are free to move
Brittleness
Ionic compounds are hard, but brittle - they shatter rather than bend when struck.
- When a force is applied, layers of ions can shift slightly and slide over one another
- This brings ions of the same charge next to each other
- Like charges repel strongly, so the layers push apart and the crystal splits
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