4.2.5 Giant Covalent Structures

Covalent lattices

• Covalent bonds are bonds between nonmetals in which electrons are shared between the atoms
• In some cases, it is not possible to satisfy the bonding capacity of a substance in the form of a molecule; the bonds between atoms continue indefinitely, and a large lattice is formed. There are no individual molecules and covalent bonding exists between all adjacent atoms
• Such substances are called giant covalent substances, and the most important examples are C and SiO₂
• Graphite, diamond, buckminsterfullerene and graphene are allotropes of carbon

Diamond

• Diamond is a giant lattice of carbon atoms
• Each carbon is covalently bonded to four others in a tetrahedral arrangement with a bond angle of 109.5^o
• The result is a giant lattice with strong bonds in all directions
• Diamond is the hardest substance known
  • For this reason it is used in drills and glass-cutting tools

The structure of diamond

Graphite

• In graphite, each carbon atom is bonded to three others in a layered structure
• The layers are made of hexagons with a bond angle of 120^o
• The spare electron is delocalised and occupies the space in between the layers
• All atoms in the same layer are held together by strong covalent bonds, and the different layers are held together by weak intermolecular forces

The structure of graphite

Buckminsterfullerene

• Buckminsterfullerene is one type of fullerene, named after Buckminster Fuller, the American architect who designed domes like the Epcot Centre in Florida
• It contains 60 carbon atoms, each of which is bonded to three others by single covalent bonds
• The fourth electron is delocalised so the electrons can migrate throughout the structure making the buckyball a semi-conductor
• It has exactly the same shape as a soccer ball, hence the nickname the football molecule

The structure of buckminsterfullerene

Graphene

• Some substances contain an infinite lattice of covalently bonded atoms in two dimensions only to form layers. Graphene is an example
• Graphene is made of a single layer of carbon atoms that are bonded together in a repeating pattern of hexagons
• Graphene is one million times thinner than paper; so thin that it is actually considered two dimensional

The structure of graphene

Silicon(IV)oxide

• Silicon(IV)oxide is also known as silicon dioxide, but you will be more familiar with it as the white stuff on beaches!
• Silicon(IV)oxide adopts the same structure as diamond -  a giant structure made of tetrahedral units all bonded by strong covalent bonds
• Each silicon is shared by four oxygens and each oxygen is shared by two silicons
• This gives an empirical formula of SiO₂

The structure of silicon dioxide

• Different types of structure and bonding have different effects on the physical properties of substances such as their melting and boiling points, electrical conductivity and solubility

Covalent bonding & giant covalent lattice structures

• Giant covalent lattices have very high melting and boiling points
  • These compounds have a large number of covalent bonds linking the whole structure
  • A lot of energy is required to break the lattice
• The compounds can be hard or soft
  • Graphite is soft as the forces between the carbon layers are weak
  • Diamond and silicon(IV) oxide are hard as it is difficult to break their 3D network of strong covalent bonds
  • Graphene is strong, flexible and transparent which it makes it potentially a very useful material

• Most compounds are insoluble with water
• Most compounds do not conduct electricity however some do
  • Graphite has delocalised electrons between the carbon layers which can move along the layers when a voltage is applied
  • Graphene is an excellent conductors of electricity due to the delocalised electrons
  • Buckminsterfullerene is a semi-conductor
  • Diamond and silicon(IV) oxide do not conduct electricity as all four outer electrons on every carbon atom is involved in a covalent bond so there are no free electrons available

Characteristics of Giant Covalent Structures Table