2.2.3The Particulate Nature of Matter
Properties of Ionic Compounds — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 7 of 13 Unit quiz
Part 1 of 4Structure that explains the properties
Giant ionic lattice
- Giant ionic lattice Ionic compounds consist of a huge, regular three-dimensional arrangement of positive and negative ions.
- Each ion is held by strong electrostatic forces of attraction to the oppositely charged ions around it.
Fixed ions in the solid
- Fixed ions in the solid In the solid the ions can only vibrate about fixed positions.
- They cannot move from place to place.
Charge matters
- Charge matters The greater the charges on the ions, the stronger the attraction, and the more energy is needed to separate them.
Part 2 of 4Properties and explanations
| Property | What is observed | Explanation |
|---|---|---|
| Melting and boiling points | High | Many strong electrostatic forces between ions must be overcome, which needs a lot of energy |
| Electrical conductivity, solid | Does not conduct | Ions are held in fixed positions and cannot move; there are no free charge carriers |
| Electrical conductivity, molten | Conducts | Ions are free to move and carry charge |
| Electrical conductivity, dissolved | Conducts | Ions are free to move in the solution |
| Solubility in water | Many dissolve | Water molecules pull ions out of the lattice and surround them |
| Hardness | Hard but brittle | Layers of ions are strongly bonded, but a shift brings like charges together and the crystal shatters |
Part 3 of 4Why conductivity depends on state
1Solid
- Solid Ions are locked in the lattice, so no current flows.
2Heat or dissolve
- Heat or dissolve Melting or dissolving breaks up the lattice and separates the ions.
3Ions free
- Ions free Positive and negative ions can now move through the liquid.
4Current flows
- Current flows The moving ions carry the electrical charge, so the compound conducts.
- Note that it is ions, not electrons, that carry the current.
Part 4 of 4Explaining a property in an exam
1
State the structure: a giant lattice of oppositely charged ions.
2
State the bonding: strong electrostatic forces act between the ions.
3
Link to the property: for melting point, say that a lot of energy is needed to overcome these forces.
4
For conductivity, say whether the ions are free to move in that state.
5
Do not mention molecules or 'breaking covalent bonds'; ionic compounds have neither.
Key terms
| Term | Meaning |
|---|---|
| Giant ionic lattice | A regular, repeating arrangement of oppositely charged ions held by electrostatic attraction. |
| Electrostatic force | The force of attraction between particles of opposite charge. |
| Molten | In the liquid state after melting. |
| Brittle | Likely to shatter when force is applied rather than bend. |
| Electrolyte | A molten or dissolved ionic compound that conducts electricity. |
| Soluble | Able to dissolve in a solvent. |
Avoid these mistakes
1
Saying that a solid ionic compound has no electrons to move. The correct reason is that ions cannot move in the solid.
2
Stating that electrons carry the current in molten or dissolved compounds. Ions carry the charge.
3
Writing that covalent bonds break when an ionic compound melts. Only the ionic attractions are overcome.
4
Stating that the ionic bond is weak because the compound is brittle. The bonds are strong; brittleness comes from layers sliding to bring like charges together.
5
Forgetting to say 'a lot of energy' when explaining high melting points.
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