Lecture Notes: Chemical Bonding Models and Material Properties

Simple Bonding Models

• Types of Bonds
  • Ionic Bonds: Occur in ionic solids; strong electrostatic forces between oppositely charged ions leading to high melting points.
  • Covalent Bonds: Strong attraction between shared pair of electrons and adjacent non-metal nuclei.
    • Non-polar Covalent: Electrons equally shared; weak intermolecular forces, low melting points.
    • Polar Covalent: Electrons not equally shared; stronger intermolecular forces, higher melting points.
  • Metallic Bonds: Attraction between metal ions and delocalized electrons; properties include ductility, malleability, high melting point, and conductivity.

Limitations of Simplified Models

• Oversimplified models do not apply universally.
• Example: Aluminium chloride is not purely ionic despite being a metal-nonmetal compound.

Bonding Continuum

• Concept: Bonds can range from ionic to non-polar covalent.
• Continuum:
  • Ionic at one end (electron transfer).
  • Non-polar covalent at the other (equal sharing).
  • Polar covalent in between (unequal sharing).

Bonding Triangle

• Purpose: Includes metallic bonds in the relationship between different bond types.
• Structure:
  • Three corners for ionic, covalent, and metallic bonds.
  • Compounds placed based on bond characteristics.

Electronegativity and Bond Prediction

• Electronegativity Difference (END): Determines bond type and properties.
• Bonding Triangle:
  • Y-axis: END
  • X-axis: Average electronegativity
• Example Calculations:
  • Magnesium chloride: 60% ionic character, high melting point.
  • Sodium chloride: More ionic, higher melting point than magnesium chloride.

Alloys

• Definition: Homogeneous mixture of two or more metals or a metal with a non-metal.
• Properties: Improved hardness, strength, corrosion resistance due to lattice distortion.
• Examples:
  • Steel: Iron and carbon mixture, stronger than iron.
  • Stainless steel: Includes chromium and nickel, corrosion-resistant.
  • Brass and bronze: Copper alloys, harder and corrosion-resistant.

Polymers

• Definition: Large molecules from joining smaller molecules (monomers).
• Types:
  • Homopolymers: Made of one type of monomer.
  • Copolymer: Made of different types of monomers.
• Properties:
  • Influenced by monomer identity, chain length, branching, and cross-links.
  • Most plastics are unreactive, hydrophobic, and strong due to covalent bonds.

Addition Polymerization

• Process: Formation of polymers by breaking pi bonds in unsaturated monomers.
• Representation: Brackets and 'n' indicate number of monomers.

Summary

• Bond types exist on a continuum with electronegativity and a bonding triangle aiding prediction of properties.
• Alloys enhance metal properties through lattice distortion.
• Polymers' properties derive from their chemical structure and polymerization process.