Knowledge Organiser: Giant Covalent Structures
Key Terms
- Giant covalent: atoms bonded in all directions — no molecules
- Allotrope: different structural form of same element
- Delocalised electron: free electron that enables conduction
- Fullerene: hollow carbon cage (C₆₀) — simple molecular
- Graphene: single layer of graphite
Must-Know Facts
- Diamond: 4 bonds per C, very hard, no conduction, transparent
- Graphite: 3 bonds per C, soft layers, conducts (free e⁻), black
- SiO₂: similar to diamond, hard, high MP, no conduction
- All giant covalent structures have very high melting points
- Graphite conducts because 1 free electron per C atom
- Graphite is soft because layers slide (weak intermolecular forces between layers)
Key Equations
- Diamond: each C bonded to 4 others in tetrahedral arrangement
- Graphite: each C bonded to 3 others; 1 delocalised electron per C
- SiO₂: each Si bonded to 4 O atoms; each O bridges 2 Si atoms
- C₆₀ (buckminsterfullerene): 60 C atoms, 20 hexagons + 12 pentagons
Common Mistakes
- Saying diamond conducts electricity: Diamond has no free electrons — all 4 outer electrons are used in covalent bonds
- Saying graphite has a high melting point because of ionic bonds: Graphite is giant covalent — high MP is due to many strong covalent bonds within layers
- Confusing graphite softness with low melting point: Graphite is soft (layers slide) but has a very high melting point (covalent bonds within layers are strong)
- Saying fullerenes are giant covalent: C₆₀ fullerene is a simple molecular structure — it has weak forces between molecules and a relatively low melting point
Memory Aid
- Diamond: 4 bonds used → ALL electrons locked → NO conduction, VERY HARD
- Graphite: 3 bonds used → 1 electron free per C → CONDUCTS, soft layers SLIDE