Giant ionic lattices (AQA GCSE Chemistry): Revision Notes
Giant ionic lattices
What are giant ionic lattices?
A giant ionic lattice is a huge 3D structure made of ions. In ionic compounds, there is strong electrostatic attraction between oppositely charged ions. These ions are arranged in a regular pattern that repeats over and over again.
The most common example you need to know is sodium chloride (table salt). It has Na+ ions and Cl- ions arranged in a giant structure.
Ionic lattices are called "giant" because they contain millions and millions of ions all bonded together in one continuous structure, not because of their physical size.
Different ways to show ionic structures
Scientists use four different types of diagrams to show ionic lattices. Each one shows different information:
Dot-and-cross diagrams
- What they show: How atoms transfer electrons to form ions
- What they don't show: How the ions are arranged in the lattice
- Example: Shows why sodium atoms form 1+ ions and chlorine atoms form 1- ions
Two-dimensional diagrams
- What they show: How ions are regularly arranged in a flat pattern
- What they don't show: The 3D structure or electronic structure of ions
- Limitation: Only shows a flat view, not the full 3D picture
Ball and stick models
- What they show:
- The giant structure of ions
- How electrostatic forces work in all directions (ionic bonding)
- What they don't show: The electronic structure of the ions
- Key point: Shows that ionic bonds link specific ions, but forces actually act in all directions
Three-dimensional models
- What they show: The actual 3D giant structure of the lattice
- What they don't show: The electronic structure of the ions
- Best for: Understanding the real shape and arrangement of the compound
Remember that no single diagram type shows everything! Each type of diagram has its own purpose and limitations. You need to understand what information each type provides and what it cannot show.
Properties of ionic compounds
Giant ionic lattices have three key properties you must remember:
High melting points
- Ionic compounds need lots of energy to melt
- This is because the electrostatic forces between ions are very strong
- You need high temperatures to break these strong bonds
High boiling points
- Similar to melting - lots of energy needed to break the strong bonds
- The strong electrostatic attraction must be overcome
Electrical conductivity
- In solid form: Do NOT conduct electricity
- When melted or dissolved in water: DO conduct electricity
- Why: In solids, ions cannot move freely. When melted or dissolved, ions can move and carry electrical current
Key Rule for Electrical Conductivity: Ionic compounds only conduct electricity when their ions are free to move. In the solid state, ions are held in fixed positions and cannot move to carry current.
Worked Example: Identifying Ionic Compounds
If you see a diagram showing Ca²⁺ and F⁻ ions arranged in a 3D structure:
- Name: Calcium fluoride
- Type of structure: Giant ionic lattice
To make calcium fluoride conduct electricity, you could:
- Melt the solid, OR
- Dissolve the solid in water
This frees the ions so they can move and carry electrical current.
Key Points to Remember:
- Giant ionic lattices are huge 3D structures of oppositely charged ions
- Strong electrostatic forces hold the ions together in regular patterns
- High melting and boiling points because strong bonds need lots of energy to break
- Only conduct electricity when ions are free to move (melted or dissolved)
- Sodium chloride is the key example you need to know for your exam