Giant Covalent Structures (DP IB Chemistry): Revision Note

Giant covalent structures

• Some substances form large networks of atoms joined by strong covalent bonds

  • These are known as covalent network structures or giant covalent lattices

  • They do not form individual molecules because the covalent bonding extends throughout the entire lattice

• Examples include:

  • Silicon

  • Silicon dioxide

  • Diamond

  • Graphite

  • Buckminsterfullerene

  • Graphene

Covalent network structures - silicon

Silicon

• In the silicon covalent network structure, each silicon atom is covalently bonded to four others

• The atoms are arranged in a tetrahedral geometry, with bond angles of approximately 109.5°

Silicon(IV) oxide

• Silicon(IV) oxide is also known as silicon dioxide

  • It is the main component of sand

• Silicon(IV) oxide forms a covalent network structure where:

  • Each silicon atom is covalently bonded to four oxygen atoms

  • Each oxygen atom is bonded to two silicon atoms

• The bonding forms a tetrahedral geometry and the structure extends in all directions

• The empirical formula is SiO₂ because the structure is based on a repeating ratio rather than discrete molecules

Covalent network structures - carbon

• Carbon exists in several different forms, called allotropes

  • Each allotrope has distinct bonding and properties

Diamond

• In the diamond covalent network structure, each carbon atom is covalently bonded to four others

• The atoms are arranged in a tetrahedral geometry, with bond angles of approximately 109.5°

• The strong covalent bonds extend in all directions, making the structure extremely rigid and durable

• Diamond is very hard because breaking the structure requires breaking many strong covalent bonds

• It does not conduct electricity because all four outer electrons on each carbon atom are used in bonding

Graphite

• In graphite, each carbon atom is covalently bonded to three others in hexagonal rings arranged in flat layers

• The bond angles are 120°, consistent with trigonal planar geometry

• Graphite conducts electricity because the fourth outer electron of each carbon is delocalised and moves freely between the layers

• Graphite is soft and slippery because the layers are held together by weak intermolecular forces and can slide over each other

Buckminsterfullerene

• Buckminsterfullerene (C₆₀) is a molecular form of carbon made of 60 atoms joined in a spherical structure 

• Each carbon forms three covalent bonds creating a pattern of interlocking hexagons and pentagons

  • The molecule is shaped like a football (soccer ball), which is why it is sometimes called the football molecule

• The remaining electron on each carbon is delocalised, allowing limited electron movement through the structure

  • So, buckminsterfullerene is a semiconductor

  • It can conduct electricity, but not as well as graphite or graphene

Graphene

• Graphene is a single layer of carbon atoms arranged in a hexagonal lattice

• Each carbon atom is bonded to three others with trigonal planar geometry and bond angles of 120°

• Graphene conducts electricity because the fourth outer electron of each carbon is delocalised

• The structure of graphene extends in two dimensions only

  • It is effectively one atom thick (about one million times thinner than paper)

  • It is strong and flexible despite its low thickness

Properties of giant covalent structures

• The structure and bonding of covalent network solids determine their physical properties, such as:

  • Melting and boiling points

    • Giant covalent lattices have very high melting and boiling points

    • They contain a large number of strong covalent bonds

    • These require large amounts of energy to break the lattice and melt the substance

  • Hardness

    • Diamond and silicon dioxide are very hard because of their 3D covalent networks

    • Graphite is soft because the layers are held by weak forces and can slide

    • Graphene is extremely strong, thin and flexible

  • Electrical conductivity

    • Most covalent network solids do not conduct electricity because all electrons are used in bonding

    • Graphite and graphene conduct electricity due to delocalised electrons

    • Buckminsterfullerene is a semiconductor

  • Solubility

    • Most covalent network structures are insoluble in water

Summary table of characteristics of giant covalent structures