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 4 parts Lesson 40 of 50
Section 1 of 4Giant Ionic Lattice
An ionic compound is a giant structure of oppositely charged ions, not a set of separate molecules.
Ionic bond
- The strong electrostatic attraction between oppositely charged ions, which acts in all directions within the lattice.
Lattice arrangement
- A regular, repeating three-dimensional arrangement of alternating positive and negative ions packed tightly together.
Formula meaning
- The formula shows the simplest ratio of ions, because the lattice has no separate molecules.
Size of charge
- Ions with larger charges attract each other more strongly, so the lattice needs more energy to break apart.
Sodium chloride
- In sodium chloride each sodium ion is surrounded by six chloride ions, and each chloride by six sodium ions.
Section 2 of 4Melting Points
1
Heating an ionic solid transfers thermal energy to the ions, making them vibrate more strongly within the lattice.
2
A large amount of thermal energy is needed to overcome the strong electrostatic forces acting in all directions.
3
The melting point rises when the ions carry larger charges, because the forces between them are stronger.
4
For example, magnesium oxide, containing 2+ and 2− ions, melts at about 2850 °C, far above sodium chloride, which melts at about 800 °C.
Section 3 of 4Conducting Electricity
Conducting Electricity
Solid state
- Ions are fixed in position in the lattice, so they cannot move and carry charge, and the solid does not conduct.
Molten state
- Heating above the melting point frees the ions, which move towards electrodes and carry the current.
Aqueous solution
- Dissolving in water separates the ions, which can then move through the solution and conduct a current.
| Property | Ionic compound | Simple covalent compound |
|---|---|---|
| Bonding | Electrostatic attraction between ions | Covalent bonds inside molecules |
| Melting point | High | Low |
| Conducts when solid? | No | No |
| Conducts when molten? | Yes | No, as there are no ions |
Section 4 of 4Exam Checklist & Tips
1.6.5 – Key Points for the Exam
- Giant ionic lattice A regular structure of many positive and negative ions held together by electrostatic forces.
- Electrostatic attraction The force of attraction between particles with opposite charges.
- Molten Melted by heating, so that the ions are free to move.
- Aqueous Dissolved in water.
- Thermal energy Energy transferred as heat, which increases the vibration of particles.
Common Mistakes
- Saying ionic compounds conduct because electrons move;
- in molten and dissolved ionic compounds it is the ions that carry the charge.
- Describing an ionic bond as a shared pair of electrons, which is the description for covalent bonds.
- Explaining a high melting point by saying bonds inside ions break; the forces between ions must be overcome.
- Referring to a molecule of sodium chloride; the lattice contains ions in a fixed ratio.
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