03.05 Giant Structures

1. Overview of Giant Structures

a. Definition:

• Giant Structures: Also known as giant lattices, these are extensive networks of atoms or ions held together by strong bonds, repeating in all directions. They encompass three main types:
  • Giant Ionic Lattices
  • Giant Covalent Structures
  • Giant Metallic Lattices

b. Importance:

• Understanding giant structures is crucial for explaining the physical properties of various substances, including hardness, melting points, electrical conductivity, and more.

2. Giant Ionic Lattice Structures

a. Structure:

• Composition: Consist of alternating positive (cations) and negative (anions) ions arranged in a regular, repeating pattern.
• Nearest Neighbours: Each ion is surrounded by ions of opposite charge. For example, in sodium chloride (NaCl):
  • Sodium ions (Na⁺): Each Na⁺ is surrounded by six chloride ions (Cl⁻).
  • Chloride ions (Cl⁻): Each Cl⁻ is surrounded by six sodium ions (Na⁺).
• Electrical Neutrality: The overall structure is electrically neutral, with equal numbers of positive and negative ions.

b. Properties:

• Hardness and Brittleness:
  • Hard: Due to the strong electrostatic forces holding the ions in place.
  • Brittle: When layers are forced to slide, ions of the same charge come close, causing repulsion and fracturing the crystal.
• High Melting and Boiling Points: Significant energy is required to overcome the strong electrostatic forces between ions.
• Solubility in Water: Many ionic compounds dissolve in water as water molecules surround and separate the ions, disrupting the lattice.
• Electrical Conductivity:
  • Solid State: Do not conduct electricity (ions are fixed in place).
  • Molten or Dissolved State: Conduct electricity (ions are free to move).

c. Examples:

• Sodium Chloride (NaCl): Common table salt.
• Magnesium Oxide (MgO): Formed by the transfer of two electrons from magnesium to oxygen.
• Calcium Chloride (CaCl₂): Requires the transfer of two electrons from calcium to two chlorine atoms.

d. Diagrams:

• Dot-and-Cross Diagrams: Illustrate the transfer of electrons from metal to non-metal, forming cations and anions.
• Ionic Lattice Diagram: Shows the regular arrangement of cations and anions in a three-dimensional lattice.

e. Summary Table: Ionic Compounds vs. Covalent Compounds

3. Giant Covalent Structures

a. Definition:

• Giant Covalent Structures: Comprise vast networks of atoms bonded together by strong covalent bonds, extending in all directions.

b. Examples:

1. Diamond (Carbon):
   • Structure: Each carbon atom is tetrahedrally bonded to four other carbon atoms, forming a rigid three-dimensional network.
   • Properties:
     • Very High Melting Point: Due to the extensive covalent bonding.
     • Extremely Hard: The strong bonds make diamond the hardest natural substance.
     • Brittle: Despite hardness, the rigid structure makes it brittle.
     • Electrical Conductivity: Does not conduct electricity (no free electrons).
   • Uses: Cutting tools, jewelry.
2. Graphite (Carbon):
   • Structure: Consists of layers of carbon atoms arranged in hexagons. Within each layer, each carbon is bonded to three others, with one electron free per carbon.
   • Properties:
     • High Melting Point: Similar to diamond due to strong in-layer bonds.
     • Soft and Slippery: Layers can slide over each other easily.
     • Electrical Conductivity: Conducts electricity due to delocalised electrons.
   • Uses: Pencils, lubricants, electrodes in electric motors.
3. Silicon(IV) Oxide (SiO₂):
   • Structure: Each silicon atom is bonded to four oxygen atoms, and each oxygen is bonded to two silicon atoms, forming a network similar to diamond.
   • Properties:
     • Very High Melting Point: Strong covalent bonds throughout the structure.
     • Hard: Rigid lattice similar to diamond.
     • Electrical Conductivity: Does not conduct electricity.
   • Uses: Glass, sand, quartz.

c. Properties:

• High Melting and Boiling Points: Result from the strong covalent bonds throughout the structure.
• Hardness: Due to the extensive bonding.
• Electrical Conductivity: Generally do not conduct electricity (except graphite).
• Brittleness: Rigid structures can fracture under stress.

d. Summary Table: Diamond vs. Graphite vs. Silicon(IV) Oxide

4. Giant Metallic Lattices

a. Definition:

• Giant Metallic Lattices: Consist of positive metal ions surrounded by a “sea” of delocalised (free-moving) electrons. This structure is held together by metallic bonding.

b. Structure:

• Metal Ions: Packed closely in a regular, repeating arrangement.
• Delocalised Electrons: Free to move throughout the lattice, creating a “sea” that holds the metal ions together.

c. Properties:

• High Melting and Boiling Points: Due to the strong attraction between the delocalised electrons and positive metal ions.
• Electrical Conductivity: Excellent conductors of electricity as free electrons can move and carry charge.
• Malleability and Ductility:
  • Malleable: Can be hammered or pressed into sheets.
  • Ductile: Can be drawn into wires.
  • Reason: Layers of metal ions can slide over each other without breaking the metallic bonds, as the delocalised electrons maintain the bond.

d. Examples:

• Copper (Cu): Highly conductive metal used in electrical wiring.
• Iron (Fe): Used in construction and manufacturing.
• Alloys (e.g., Brass, Bronze): Mixtures of metals with enhanced properties.

e. Diagram: Metallic Bonding

+     +     +     +
 \   / \   / \   / \
  e⁻ e⁻ e⁻ e⁻ e⁻ e⁻
 /   \ /   \ /   \ 
+     +     +     +

• Positive Ions (+): Represent metal ions.
• e⁻: Represent delocalised electrons.

f. Summary Table: Metallic Compounds vs. Ionic and Covalent Compounds

5. Comparison of Giant Structures

6. Practice Questions

a. Question 8: Which of the following elements exists as a giant covalent structure?

• A. Carbon
• B. Iodine
• C. Helium
• D. Oxygen

Answer:

• A. Carbon
• Explanation: Carbon exists as diamond (a giant covalent structure) and graphite (a layered giant covalent structure).

b. Question 9: Carbon and silicon are both in Group IV of the Periodic Table. Using Table 3.7, which row (A–D) contains the text that best describes the bonding structure of carbon dioxide and silicon(IV) oxide?

Answer:

• C.
  • Explanation: Carbon dioxide has molecules held together by weak intermolecular forces, while silicon(IV) oxide forms a giant covalent lattice with strong covalent bonds.

c. Question 10: Metals such as copper are bonded together by the attraction between metal ions and a “sea” of delocalised electrons surrounding them. Which of the properties of metals (A–D) is not explained by this type of bonding?

• A. Electrical conductivity
• B. Malleability
• C. Melting point
• D. Reaction with acids

Answer:

• D. Reaction with acids
• Explanation:
  • Reaction with acids is not directly explained by metallic bonding. It involves the chemical reactivity of the metal with acids, which can depend on factors like the metal’s position in the reactivity series rather than just the metallic bonding.

7. Key Terminology

• Giant Ionic Lattice: A three-dimensional network of alternating positive and negative ions held together by strong electrostatic forces.
• Giant Covalent Structures: Extensive networks of atoms bonded by covalent bonds, extending in all directions.
• Giant Metallic Lattices: Regular arrangement of positive metal ions surrounded by a sea of delocalised electrons.
• Cation: A positively charged ion formed by the loss of electrons.
• Anion: A negatively charged ion formed by the gain of electrons.
• Delocalised Electrons: Electrons that are free to move throughout the metallic lattice, facilitating electrical conductivity.
• Malleable: Ability of a metal to be hammered or pressed into sheets.
• Ductile: Ability of a metal to be drawn into wires.
• Electrostatic Forces: Forces of attraction between oppositely charged ions.

8. Activity: Understanding Giant Structures

Objective: To visually comprehend the differences between giant ionic, covalent, and metallic structures through diagrams and comparisons.

Instructions:

1. Draw Structural Diagrams:
   • Giant Ionic Lattice: Illustrate NaCl lattice showing alternating Na⁺ and Cl⁻ ions.
   • Giant Covalent Structure (Diamond): Show tetrahedral bonding between carbon atoms.
   • Giant Metallic Lattice: Depict metal ions surrounded by a sea of delocalised electrons.
2. Compare Properties:
   • Create a comparison chart highlighting how bonding type influences properties like melting point, hardness, conductivity, etc.
3. Real-World Examples:
   • Identify and list real-world materials that exemplify each giant structure type (e.g., table salt for giant ionic, diamond for giant covalent, copper for giant metallic).
4. Discussion:
   • Discuss how the structure of each type influences its use in everyday applications.