2.6 BETA Verified

Giant covalent structures

4 learning objectives • 2 core • 2 extended

2.6 Giant Covalent Structures Revision Notes

1. Overview

Giant covalent structures, also known as macromolecules, consist of a vast network of atoms held together by strong covalent bonds extending throughout the entire structure. Unlike simple molecular substances, these structures do not have a fixed number of atoms, resulting in exceptionally high melting points and distinct physical properties that make them essential for industrial applications.

Key Definitions

  • Giant Covalent Structure: A three-dimensional network of atoms joined together by many strong covalent bonds.
  • Allotrope: Different structural forms of the same element in the same physical state (e.g., diamond and graphite are allotropes of carbon).
  • Delocalized Electron: An electron that is not associated with a single atom or covalent bond and is free to move throughout a structure.
  • Macromolecule: A very large molecule, such as a polymer or a giant covalent structure.

Core Content

Diamond (Allotrope of Carbon)

  • Structure: Each carbon atom is joined to four other carbon atoms by strong covalent bonds in a rigid tetrahedral arrangement.
  • Properties:
    • Extremely Hard: Due to the rigid network of strong covalent bonds.
    • High Melting/Boiling Point: Large amounts of energy are required to break the numerous strong covalent bonds.
    • Does Not Conduct Electricity: All four outer-shell electrons are used in bonding; there are no free electrons to move and carry charge.
  • Use in Cutting Tools: Because of its extreme hardness, diamond is used on the tips of drills and glass cutters to cut through tough materials.
📊A 3D tetrahedral lattice where each sphere (carbon) is bonded to four others.

Graphite (Allotrope of Carbon)

  • Structure: Each carbon atom is joined to three other carbon atoms, forming layers of hexagonal rings.
  • Bonding: The fourth electron from each carbon atom is delocalized and can move between the layers.
  • Intermolecular Forces: There are weak forces of attraction between the layers, allowing them to slide over each other.
  • Properties:
    • Soft and Slippery: The layers can easily slide over each other due to weak forces between them.
    • Conducts Electricity: The delocalized electrons are free to move throughout the structure and carry a current.
  • Uses:
    • Lubricant: Its slippery nature makes it ideal for reducing friction in machinery.
    • Electrodes: Because it conducts electricity and has a high melting point, it is used in electrolysis.
📊Layers of hexagonal rings of carbon atoms with dotted lines between layers representing weak forces.

Extended Content (Extended Curriculum Only)

Silicon(IV) Oxide (Silica, SiO₂)

  • Structure: Silicon(IV) oxide occurs naturally as quartz. Each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms.
  • Similarity to Diamond: The structure of SiO₂ is very similar to diamond. It forms a vast, rigid tetrahedral lattice.
📊A 3D lattice showing Silicon atoms bonded to four Oxygen atoms in a tetrahedral shape.

Comparison of Properties: Diamond vs. Silicon(IV) Oxide

Because they share similar giant covalent tetrahedral structures, they share several physical properties:

  1. Hardness: Both are very hard materials because of the strong covalent bonds throughout the lattice.
  2. High Melting Points: Both require massive amounts of heat energy to break the strong bonds before they can melt.
    • Melting point of Diamond: ~3550 °C
    • Melting point of SiO₂: ~1710 °C
  3. Electrical Insulators: Neither has delocalized electrons or ions (in solid state) to carry electrical charge.

Key Equations

While giant covalent structures are defined by their bonding, they can undergo chemical reactions such as combustion.

Combustion of Carbon (Diamond or Graphite):

  • Word Equation: Carbon(s) + Oxygen(g) → Carbon dioxide(g)
  • Symbol Equation: C(s) + O₂(g) → CO₂(g)

Note on Silica (SiO₂): Silicon(IV) oxide is chemically relatively inert but can react with strong bases at high temperatures:

  • Word Equation: Silicon(IV) oxide(s) + Sodium hydroxide(aq) → Sodium silicate(aq) + Water(l)
  • Symbol Equation: SiO₂(s) + 2NaOH(aq) → Na₂SiO₃(aq) + H₂O(l)

Common Mistakes to Avoid

  • ❌ Wrong: Saying graphite conducts electricity because it has "free ions."
    • ✓ Right: Graphite conducts because it has delocalized electrons. Only ionic compounds conduct via ions (when molten/aqueous).
  • ❌ Wrong: Describing diamond as having "intermolecular forces" that break when it melts.
    • ✓ Right: Diamond has no molecules; you must break strong covalent bonds to melt it.
  • ❌ Wrong: Thinking Silicon(IV) oxide (SiO₂) is a simple molecule like CO₂.
    • ✓ Right: SiO₂ is a giant covalent structure, whereas CO₂ is a simple molecular gas.

Exam Tips

  • Command Words: If asked to "Describe the structure," mention the arrangement of atoms and the type of bonding (e.g., "tetrahedral arrangement of carbon atoms held by covalent bonds").
  • Comparison Questions: If asked to compare graphite and diamond, create a table highlighting: number of bonds per carbon (3 vs 4), presence of delocalized electrons, and hardness.
  • Context: You may see SiO₂ referred to as "sand" or "quartz." Remember that its properties (high melting point/hardness) are always due to its giant covalent structure.
  • Properties to Structure: Always link the property to the structure.
    • Slippery? → "Layers can slide."
    • Conducts? → "Delocalized electrons move."
    • Hard? → "Strong covalent bonds in a rigid lattice."