Giant Covalent Structures

Structure of Giant Covalent Compounds

• Definition:
  • A giant covalent structure is a continuous three-dimensional network of atoms held together by strong covalent bonds.
  • Unlike simple covalent molecules, giant covalent structures do not consist of separate molecules; instead, the atoms are bonded in a huge lattice.

Graphite

Structure

• Carbon atoms arranged in layers.
• Each carbon atom is covalently bonded to 3 other carbon atoms in a hexagonal arrangement.
• The 4th valence electron of each carbon is delocalised and free to move.
• Layers are held together by weak intermolecular forces (Van der Waals’ forces).
• Layers can slide over each other easily because of weak forces between them.

Diagram (Dot-and-Cross Notation):

Layer 1:  C—C—C—C—C  
          |  |  |  |  
Layer 2:  C—C—C—C—C  
(weak forces between layers)

Properties and Explanations

1. Good conductor of electricity:
   • Delocalised electrons can move freely along the layers, carrying charge.
2. Soft and slippery:
   • Weak forces between layers allow them to slide over one another easily.
3. High melting point:
   • Strong covalent bonds within layers require a lot of energy to break.
4. Insoluble in water and organic solvents:
   • Covalent network is too strong to be broken by solvent molecules.

Uses

• Lubricant: Layers slide easily, reducing friction in machinery.
• Electrodes in batteries and electrolysis: Delocalised electrons make it a good conductor, and it is chemically unreactive at high temperatures.

Diamond

Structure

• Each carbon atom is covalently bonded to 4 other carbon atoms in a tetrahedral arrangement.
• Structure forms a rigid 3D lattice.
• No free electrons – all valence electrons are used in covalent bonds.

Diagram (Tetrahedral bonding):

    C
   /|\
  C C C
 / \
C   C

Properties and Explanations

1. Very hard:
   • Rigid 3D lattice with strong covalent bonds throughout the structure.
2. High melting and boiling points:
   • Large amount of energy required to break strong covalent bonds.
3. Does not conduct electricity:
   • No free or delocalised electrons.
4. Transparent and colourless:
   • Symmetrical structure does not absorb visible light significantly.

Uses

• Cutting tools and drills: Extreme hardness makes diamond ideal for cutting hard materials.
• Jewellery: Attractive luster, brilliance, and rarity.

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course

Silicon(IV) Oxide (SiO₂)

Structure

• Also called silica.
• Each silicon atom is covalently bonded to four oxygen atoms.
• Each oxygen atom is covalently bonded to two silicon atoms.
• Forms a giant tetrahedral lattice, similar to diamond.

Diagram (Simplified):

   O
  / \
 Si  O
 / \
O   Si

Properties and Explanations

1. Hard and strong:
   • Strong covalent bonds throughout the structure.
2. High melting and boiling points:
   • Large amount of energy required to break the covalent network.
3. Does not conduct electricity:
   • No delocalised electrons or mobile ions.
4. Insoluble in water:
   • Strong covalent bonds not broken by solvent molecules.

Uses

• Glass manufacturing: Transparent, hard, and heat-resistant.
• Ceramics: Hardness and resistance to chemical attack.
• Construction material: Found in sand and quartz for building materials.

Comparison Table

Similarities between Diamond and Silicon(IV) Oxide

• Giant tetrahedral structures with each atom bonded to 4 others.
• Strong covalent bonds throughout the structure.
• Very high melting and boiling points.
• Do not conduct electricity (no free electrons).
• Hard and strong.