Giant Covalent Structures (SL IB Chemistry)

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
    
    

  Diagram to show the tetrahedral structure of diamond

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

Diagram to show the layered structure of graphite

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

Diagram to show the interlocking hexagons and pentagons that make up the structure of Buckminsterfullerene

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

Diagram to show the two dimensional structure of graphene

The structure of graphene

Silicon

• The silicon atoms in silicon have a tetrahedral arrangement, just like that of the carbon atoms in diamond
• Each silicon atom is covalently bonded to four other silicon atoms 
• Silicon has a giant lattice structure 

Diagram to show the tetrahedral arrangement in silicon

The structure of silicon

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 silicon atoms
• This gives an empirical formula of SiO₂

Diagram to show the tetrahedral units in silicon(IV) oxide

The structure of silicon dioxide

Properties of Giant Covalent Structures

• 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

• 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

	Diamond	Graphite	Graphene	Buckminster-fullerene	Silicon	Silicon dioxide
Melting and boiling point	Very high	Very high	Very high	Low	High	Very high
Appearance	Transparent crystal	Grey solid	Transparent	Black powder	Grey-white solid	Transparent crystals
Electrical conductivity	Non-conductor	Good	Very good	Poor	Poor	Non-conductor
Thermal conductivity	Good	Poor	Very good	Poor	Good	Good
Other properties	Hardest known natural substance	Soft and slippery	Thinnest and strongest material to exist	Light and strong	Good mechanical strength	Piezoelectric—produces electric charge from mechanical stress