Vander Waals Forces and London Dispersion Forces

Basic Definition

• Vander Waals forces: attractive forces between molecules
• London dispersion forces: a type of Vander Waals force occurring in nonpolar molecules

Atom Structure

• Atoms consist of a positively charged nucleus surrounded by a negatively charged electron cloud
• When electrons are evenly distributed, the atom is nonpolar
• Movement of electrons can lead to temporary polarization of the atom

Polarization and Dipoles

• Temporary Dipole: Occurs when one side of the atom has more electrons than the other, creating a net negative and positive side
• Induced Dipole: A dipole created in one atom due to the influence of a nearby temporary dipole

Polarizability

• Refers to the likelihood of an atom's electron cloud being distorted
• Greater number of electrons in an atom leads to higher polarizability
• Example: Iodine (53 electrons) is more polarizable than Fluorine (9 electrons)

Characteristics of Vander Waals Forces

• Very weak forces
• Dominant in nonpolar molecules
• Explain variations in boiling points among nonpolar molecules

Conclusion

• For hydrocarbons (only carbon and hydrogen), they are nonpolar
• Intermolecular forces acting: London dispersion forces (Vander Waals forces)
• Highest molar mass = Highest boiling point