Lecture Notes: General Forces of Attraction

Overview

• Focus on general forces of attraction observed in chemistry, particularly in intermolecular and intramolecular interactions.
• Forces of Attraction: Interactions that keep molecules together or affect physical properties.

Categories of Forces

1. Intramolecular Forces

• Definition: Forces within a molecule.
• Examples: Covalent bonds, ionic bonds.

2. Intermolecular Forces

• Definition: Forces between molecules affecting physical properties like melting and boiling points.
• Examples:
  • London dispersion forces
  • Dipole-dipole interactions
  • Ion-dipole interactions
  • Hydrogen bonds

Detailed Examination of Intermolecular Forces

London Dispersion Forces

• Description: Weak forces due to temporary dipoles in molecules.
• Example: Movement of electrons in helium creating temporary dipoles.
• Polarizability:
  • Ability of a molecule to become polar.
  • Higher polarizability means easier electron displacement.

Dipole-Dipole Interactions

• Description: Electrostatic interactions between permanent dipoles in molecules.
• Example: Interaction between iodine and chlorine where chlorine pulls electrons to become more negative.

Hydrogen Bonding

• Definition: Special type of dipole-dipole interaction.
• Conditions: Occurs when hydrogen is bonded to highly electronegative atoms (e.g., oxygen, nitrogen, fluorine).
• Characteristics:
  • Significant impact on boiling/melting points.
  • Many hydrogen bonds increase the strength of these interactions.

Intramolecular Forces

Ionic Bonding

• Description: Interaction between metals and non-metals, forming ionic compounds.
• Structure: Forms a lattice of ion pairs (e.g., sodium chloride).
• Properties: High melting points, crystalline structure, brittle nature, conductivity in molten or dissolved state.

Covalent Bonding

• Description: Sharing of electrons between atoms.
• Types of Bonds:
  • Sigma Bonds: Direct overlap of orbitals.
  • Pi Bonds: Side-to-side overlap of p orbitals.

Coordinate/Dative Bonding

• Description: One atom donates both electrons for the bond.
• Example: Ammonia and boron trifluoride interaction.

Bond Characteristics

Hybridization

• Concept: Mixing of atomic orbitals to form new hybrid orbitals.
• Types:
  • sp3: Four hybrid orbitals.
  • sp2: Three hybrid orbitals.
  • sp: Two hybrid orbitals.

Metallic Bonding

Description

• Structure: Lattice of cations with a sea of delocalized electrons.
• Properties:
  • Conductivity: Free electrons allow current flow.
  • Malleability/Ductility: Can be shaped without breaking due to electron sea.
  • Luster: Reflective due to electron interaction with light.
  • High Melting Points: Strong metallic bonds require more energy to break.

Conclusion

• Intramolecular forces govern the structure and strength within molecules.
• Intermolecular forces influence physical properties and states of matter.
• Understanding these forces is crucial for studying the stability and reactivity of different substances.