1.6.5Principles of Chemistry
Ionic compounds: Bonds, Structure and Properties — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 39 of 50
Part 1 of 4The giant ionic lattice
Regular arrangement
- Regular arrangement Ionic compounds consist of a huge number of positive and negative ions packed in a regular, repeating pattern called a giant lattice.
Strong bonds in all directions
- Strong bonds in all directions Each ion is surrounded by ions of opposite charge,
- so strong electrostatic forces of attraction act in every direction through the crystal.
Crystal shape
- Crystal shape In sodium chloride, each Na+ is surrounded by six Cl− and vice versa, giving cubic crystals.
Part 2 of 4Properties explained
| Property | Observation | Explanation |
|---|---|---|
| Melting and boiling point | High | A lot of heat energy is needed to overcome the strong electrostatic forces between ions throughout the lattice. |
| State at room temperature | Solid | Melting points are far above room temperature. |
| Conductivity as a solid | Does not conduct | The ions are held in fixed positions and cannot move, and there are no free electrons. |
| Conductivity when molten or dissolved | Conducts | The ions are free to move and carry the charge. |
| Solubility | Many dissolve in water | Water molecules pull ions out of the lattice and surround them. |
| Hardness | Hard but brittle | A blow shifts a layer so that like charges meet and repel, and the crystal splits. |
Part 3 of 4Why molten and dissolved ionic compounds conduct
1Add heat or water
- Add heat or water Melting or dissolving breaks the lattice apart.
2Ions become mobile
- Ions become mobile The ions are no longer fixed and can move around freely.
3Charge carried
- Charge carried Moving charged particles form an electric current when a voltage is applied.
4Electrolysis
- Electrolysis The ions are discharged at the electrodes, so the compound is broken down chemically.
Part 4 of 4Comparing strengths of ionic bonding
Higher charge, stronger attraction
- Higher charge, stronger attraction Ions with larger charges attract more strongly,
- so MgO, with 2+ and 2− ions, has a much higher melting point than NaCl, with 1+ and 1− ions.
Smaller ions
- Smaller ions Smaller ions can get closer together, increasing the force of attraction and the melting point.
Writing a good answer
- Writing a good answer Always refer to ions and electrostatic forces.
- Do not talk about molecules or about breaking covalent bonds.
Key terms
| Term | Meaning |
|---|---|
| Giant ionic lattice | A regular three-dimensional arrangement of alternating positive and negative ions. |
| Electrostatic force | The force of attraction between oppositely charged particles. |
| Molten | Melted; a substance in the liquid state. |
| Electrolyte | A liquid or solution containing mobile ions that conducts electricity. |
| Brittle | Breaks or shatters easily when hit rather than bending. |
Avoid these mistakes
1
Saying that covalent bonds break when an ionic compound melts. The ionic attractions between ions are overcome.
2
Saying a solid ionic compound has no charged particles; it does, but they cannot move.
3
Claiming that the electrons flow in molten ionic compounds. It is the ions that carry the current.
4
Describing ionic compounds as made of molecules.
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