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1.5.6Physical Chemistry

Giant Lattices — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 22 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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Part 1 of 5Giant Ionic Lattices

1.5.6 Giant Lattices
Structure
  • Oppositely charged ions alternate in a regular three-dimensional lattice held by strong electrostatic attractions in all directions.
High melting point
  • A large amount of energy is needed to overcome the many strong attractions between oppositely charged ions.
Conductivity
  • Solid ionic compounds do not conduct because the ions are held in place, but molten or dissolved ionic compounds do conduct.
Brittleness
  • Shifting the layers brings like charges together, so repulsion splits the crystal.
Solubility
  • Many dissolve in water because polar water molecules attract ions strongly enough to pull them from the lattice.

Part 2 of 5Giant Covalent Structures

Giant Covalent Structures
Diamond
  • Each carbon forms four covalent bonds in a tetrahedral network, making it extremely hard with a very high melting point.
Graphite
  • Each carbon bonds to three others in flat layers with one delocalised electron per atom, so graphite conducts electricity.
Graphite layers
  • Layers are held by weak London forces so they slide easily, which makes graphite soft and slippery.
Silicon dioxide
  • Each silicon is bonded to four oxygen atoms in a network, giving a hard solid with a very high melting point.

Part 3 of 5Comparing Structures

Ionic lattice
  • Strong electrostatic attraction, high melting point, conducts only when molten or aqueous.
Metallic lattice
  • Positive ions in a sea of electrons, conducts in solid state and is malleable.
Giant covalent
  • Millions of covalent bonds must be broken to melt it, so melting points are very high.
Simple molecular
  • Only weak intermolecular forces separate the molecules, so melting points are low.
StructureExampleConducts when solid?
Giant ionicNaClNo
Giant metallicCuYes
Giant covalentDiamondNo
Giant covalentGraphiteYes

Part 4 of 5Answering Structure Questions

1
Identify the type of structure from the particles present, such as ions, atoms or molecules.
2
State the bonding or force acting between those particles in the solid.
3
Link the strength of that bonding to the energy needed to melt or dissolve the substance.
4
Explain conductivity by naming the mobile charge carriers, or say that none are present.
5
Finish by connecting the property to the observation requested in the question.

Part 5 of 5Exam Checklist & Tips

1.5.6 – Key Points for the Exam

  • Giant lattice A huge three-dimensional network of particles held together by bonds throughout.
  • Ionic bond Electrostatic attraction between oppositely charged ions.
  • Allotrope Different structural forms of the same element, such as diamond and graphite.
  • Delocalised electron A mobile electron that is not tied to one atom.

Common Mistakes

  • Writing that ionic compounds contain molecules; they contain a lattice of ions.
  • Saying covalent bonds break when diamond melts but forgetting to state that a very large number must break.
  • Stating that graphite conducts because of ions; it conducts through delocalised electrons.
  • Giving ionic bonds as strong enough to conduct in the solid state; the ions cannot move.

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