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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

S2.2 - The covalent model

2.2.1 Covalent Bonds and Lewis Formulas 2.2.2 Bond Types 2.2.3 Co-coordination (Dative) Bonds 2.2.4 VSEPR Shapes of Molecules 2.2.5 Electronegativity and Bond Polarity 2.2.6 Polarity and Dipole Moments 2.2.7 Covalent Network Structures and Allotropes 2.2.8 Intermolecular Forces 2.2.9 Physical Properties of Covalent Substances 2.2.10 Chromatography and Intermolecular Forces 2.2.11 Resonance Structures (AHL) 2.2.12 Benzene and Resonance (AHL) 2.2.13 Expanded Octet and VSEPR (AHL) 2.2.14 Formal Charge (AHL) 2.2.15 Sigma and Pi Bonds (AHL) 2.2.16 Hybridization (AHL)

Covalent Network Structures and Allotropes

Specification Reference S2.2.7

Quick Notes

  • Carbon and silicon can form giant covalent networks: large, continuous bonding structures held together by covalent bonds.
  • These structures have very high melting points, are insoluble, and have unique electrical and mechanical properties.
  • Carbon allotropes:
    • Diamond: tetrahedral, very hard, insulator
    • Graphite: layers of hexagons, conducts electricity, slippery
    • Fullerenes (C60): molecular, spherical, conducts poorly
    • Graphene: single layer of graphite, excellent conductor
  • Silicon: similar to diamond in bonding — tetrahedral and hard
  • Silicon dioxide (SiO2): strong network structure, high melting point, insoluble

Full Notes

Some non-metal elements and compounds form giant covalent network structures.

These have no maximum size and all atoms are covalently bonded in a repeating pattern. Common examples include diamond, graphite and silicon dioxide (see below).

Carbon Allotropes

Allotropes are different structural forms of the same element, where atoms are bonded in different arrangements. Because of this, allotropes have unique chemical and physical properties, such as hardness, conductivity, colour, or reactivity.

For example, carbon exists as diamond (hard and transparent) and graphite (soft and conductive), even though both are made entirely of carbon atoms.

Diamond

IB Chemistry diagram of diamond lattice showing tetrahedral sp3 carbon network in a giant covalent structure.

Each carbon atom bonds to four others in a 3D tetrahedral lattice with no ‘maximum’ size (carbon atoms can keep bonding to more carbon atoms)

Graphite

IB Chemistry diagram of graphite showing layers of hexagonal carbon sheets with delocalised electrons and weak interlayer forces.

Each carbon atom bonds to three others, forming hexagonal layers with delocalised electrons between the layers and weak forces of attraction exist between layers.

Fullerenes (e.g. C60)

IB Chemistry diagram of fullerene C60 showing spherical molecular structure composed of pentagons and hexagons.

Carbon atoms bond together to form molecules with spherical shapes or tubes (e.g. buckyballs, nanotubes). Fullerenes are molecular substances (not a network like graphite or diamond)

Graphene

IB Chemistry diagram of graphene showing a single hexagonal layer of sp2 carbon with fully delocalised electrons.

A single layer of graphite (one-atom-thick sheet of hexagons).

Each carbon bonded to 3 others with fully delocalised electrons.

Properties:

Silicon and Silicon Dioxide

Silicon

IB Chemistry diagram of crystalline silicon showing tetrahedral covalent network analogous to diamond.

Like carbon in diamond, each atom forms 4 covalent bonds in a tetrahedral structure, forming a giant covalent network

Properties:

Silicon Dioxide (SiO2)

IB Chemistry diagram of silicon dioxide network showing each silicon bonded to four oxygens and each oxygen to two silicons.

Similar structure to diamond, but:

Found in quartz and sand.

Properties:

Summary: Structure and Properties Table

Substance Bonding / Structure Electrical Conductivity Melting Point Mechanical Properties Notes
Diamond (C) Giant covalent network; each C is sp3 and bonded to 4 C in a tetrahedral lattice Insulator (no mobile electrons or ions) Very high Extremely hard; not slippery Strong σ bonds in all directions
Graphite (C) Layered hexagonal sheets; each C is sp2 with delocalised electrons between layers Good conductor (within layers) Very high Soft and slippery (layers slide) Weak forces between layers; lubricating
Graphene (C) Single sheet of sp2 hexagons; fully delocalised π system Excellent conductor Very high (decomposes before melts) Extremely strong and flexible One-atom-thick; high surface area
Fullerenes (e.g. C60) Molecular (discrete cages/tubes) Poor conductor Lower than diamond/graphite Soft Van der Waals forces between molecules
Silicon (Si) Giant covalent network; tetrahedral like diamond Semiconductor High Hard Conductivity improved by doping
Silicon dioxide (SiO2) Giant covalent network; each Si to 4 O, each O to 2 Si Insulator High Hard; insoluble Found in quartz and sand

Summary