CHEMICAL BONDING - 2

I. Factors Influencing Intermolecular Forces

1. Electronegativity

• Definition: Electronegativity is the tendency of an atom to attract electrons towards itself when it forms a chemical bond. The scale commonly used is the Pauling scale, which ranks elements based on their ability to attract electrons.
• Impact on Intermolecular Forces:
  • Dipole-Dipole Interactions: In polar molecules, the difference in electronegativity between bonded atoms creates a dipole moment, where one end of the molecule becomes partially positive and the other end partially negative. This polarity allows for stronger dipole-dipole interactions between molecules.
  • Hydrogen Bonding: Particularly strong hydrogen bonds occur when hydrogen is bonded to highly electronegative atoms (like O, N, or F). The large electronegativity difference leads to significant polarity, enhancing the attraction between adjacent molecules.

2. Polarizability

• Definition: Polarizability refers to the ease with which the electron cloud around an atom or molecule can be distorted by an external electric field, resulting in a temporary dipole.
• Impact on Intermolecular Forces:
  • London Dispersion Forces: Larger atoms and molecules, with more electrons and diffuse electron clouds, are more polarizable. As a result, they can induce temporary dipoles in neighboring molecules, leading to stronger London dispersion forces.
  • Correlation with Molecular Size: In a group of similar molecules, those with larger atomic or molecular sizes tend to have greater polarizability, thus stronger dispersion forces.

3. Size and Shape

• Definition: The size refers to the dimensions of the molecule, while shape pertains to the spatial arrangement of atoms within the molecule.
• Impact on Intermolecular Forces:
  • Surface Area: Larger surface areas in elongated or branched molecules increase the area available for intermolecular interactions, enhancing London dispersion forces.
  • Steric Effects: The shape of molecules can lead to steric hindrance, which may reduce the effectiveness of dipole-dipole interactions or hydrogen bonds if the molecular geometry prevents optimal orientation.

4. Charge

• Definition: Charge refers to the presence of positive or negative electrical charges in ions or charged molecules.
• Impact on Intermolecular Forces:
  • Ionic Interactions: Charged species (ions) experience strong electrostatic attractions to oppositely charged ions, leading to significantly stronger interactions than those found in neutral molecules.
  • Influence on Solubility: Charged molecules can greatly affect solubility in polar solvents (e.g., salts dissolving in water), as interactions between solvent molecules and ions are favorable.

5. Dipole Moment

• Definition: The dipole moment is a quantitative measure of the polarity of a molecule, defined as the product of the charge and the distance between the charges.
• Impact on Intermolecular Forces:
  • Strength of Interactions: Molecules with larger dipole moments exhibit stronger dipole-dipole interactions, as the permanent charge separation enhances attractions between adjacent polar molecules.
  • Contribution to Hydrogen Bonding: A higher dipole moment in a hydrogen-bond donor molecule (like water) strengthens hydrogen bonds with acceptor molecules (like ammonia).

6. Temperature

• Definition: Temperature is a measure of the average kinetic energy of the particles in a substance.
• Impact on Intermolecular Forces:
  • Kinetic Energy: Increased temperature raises the kinetic energy of molecules, which can overcome intermolecular forces. This leads to decreased solubility of gases in liquids and can change phase states (e.g., melting, boiling).
  • Effect on Viscosity: Higher temperatures typically decrease viscosity in liquids, as increased molecular motion allows for easier flow despite intermolecular attractions.

7. Pressure

• Definition: Pressure is the force exerted by molecules in a given area, which can influence their proximity.
• Impact on Intermolecular Forces:
  • Increased Density: Higher pressures push molecules closer together, enhancing the effectiveness of intermolecular forces, particularly in gases where intermolecular distances are larger than in liquids or solids.
  • Phase Changes: Increased pressure can also facilitate phase transitions, such as gases becoming liquids at lower temperatures due to enhanced intermolecular interactions.

8. Molecular Weight

• Definition: Molecular weight is the mass of a molecule, often correlated with the number of atoms present and their types.
• Impact on Intermolecular Forces:
  • Correlation with Polarizability: Generally, heavier molecules are larger and more polarizable, resulting in stronger London dispersion forces.
  • Boiling and Melting Points: In a homologous series, as molecular weight increases, so does boiling and melting points due to increased intermolecular forces associated with larger size and polarizability.