2.2.7—Covalent network structures
- Syllabus
- First assessment 2025
- Objective
- 2.2.7
- Level
- SL
Covalent Network Structures
| Material | Structural evidence | Property or use explained |
|---|---|---|
| Diamond | each C covalently bonded in a rigid 3D network | very hard; high melting point; no mobile charge carriers |
| Graphite | strong covalent sheets with delocalized electrons; weak attractions between sheets | conducts along sheets; layers slide, so it is soft/lubricating |
| Graphene | one atom-thick covalent sheet with delocalized electrons | strong, light and electrically conducting |
| Fullerenes | finite carbon cages or tubes rather than an infinite 3D network | molecular shape and intermolecular contacts give properties distinct from diamond/graphite |
| Silicon | extended covalent structure with limited charge mobility | semiconductor behaviour; detailed doping is outside this card |
| Silicon dioxide | 3D Si–O covalent network, not discrete SiO₂ molecules | hard and high-melting because many strong covalent bonds must be overcome |
Decide conductivity by available mobile charges, not by the word covalent alone.
Explain a network material property by connecting the structure and bonding arrangement to the relevant mobility, strength, or dimensional feature.
Diamond is hard because each carbon is held in a three-dimensional covalent network, while graphite conducts along layers through delocalized electrons and its layers can slide. Silicon dioxide is also an extended network: describe network atoms, not discrete SiO₂ molecules, when explaining its high melting point.
Comparing Network Materials
Assessment in practice
Representative question
Question 1
[Maximum number: 4]
Identify three allotropes of carbon and describe their structures.
Allotropes:
diamond
graphite
fullerene
graphene;
Structures:
Diamond:
tetrahedral arrangement of (carbon) atoms/each carbon bonded to four others / sp3 and 3D/covalent network structure;
Graphite:
each carbon bonded to three others (in a trigonal planar arrangement) / sp2 and 2D / layers of (carbon) atoms;
Fullerene:
each (carbon) atom bonded to three others (in a trigonal arrangement) / sp2 and joined in a ball/cage/sphere/connected hexagons and pentagons;
Graphene:
each carbon bonded to three others (in a trigonal arrangement) / sp2 and 2D structure;
The Covalent Model Summary
Retrieve the covalent pathway: shared pairs and bond order lead to geometry, polarity and molecular polarity; structure determines network properties, IMF behaviour and chromatography; HL representations extend to resonance, formal charge, sigma/pi bonds and hybridization.
Check the representation first, then count domains, apply geometry, identify polarity or forces, and connect the structure to the requested property or HL bonding description.