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Attractions Between Molecules orIntermolecular Forces (IM Forces)
Chapter 8.4
Learning Objectives
• Understand the difference between intramolecular and intermolecular forces
• Identify the 4 common types of intermolecular forces
We have already talked about intramolecular forces.
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
Covalent bonds would be an example.
Intermolecular Forces
Intramolecular = strong
Intermolecular = weak
They do control physical properties such as boiling and melting points, vapor pressure, and viscosity
Types of Intermolecular Forces
• Dipole-dipole interactions
• Ion-Dipole interactions
• London dispersion forces
Van der Waals Forces
Hydrogen bonding
Dipole-Dipole Interactions
Molecules that have permanent dipoles are attracted to each other.
Ion-Dipole InteractionsAttractive forces between an ion and a polar molecule
Ion-Dipole Interaction
The larger the charge the stronger the force
London Dispersion Forces
Occasionally electrons wind up on the same side of the atom.
London Dispersion Forces
At that instant, the helium atom is polar.
London Dispersion Forces
This polar helium atom then induces a dipole
on a neighboring helium atom.
London Dispersion Forces
London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole.
London Dispersion Forces
• These forces are present in all molecules, whether they are polar or nonpolar.
• The tendency of an electron cloud to distort in this way is called polarizability.
Types of Intermolecular Forces
• The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong.
• We call these interactions hydrogen bonds.
Hydrogen Bonding
Hydrogen Bonding: Water
Hydrogen Bonding: Water