Attractions Between Molecules or Intermolecular Forces (IM Forces) Chapter 8.4

Learning Objectives • Understand the difference between intra molecular and inter molecular forces • Identify the 4 common types of intermolecular forces

We have already talked about

intramolecular forces

.

Intramolecular forces

hold atoms together in a molecule .

Covalent bonds would be an example.

Intermolecular forces

are attractive forces

between

molecules.

Intermolecular Forces Int ra molecular = strong Int er molecular = weak They do control physical properties such as boiling and melting points, vapor pressure, and viscosity

Types of Intermolecular Forces Van der Waals Forces • Dipole-dipole interactions • Ion-Dipole interactions • London dispersion forces Hydrogen bonding

Dipole-Dipole Interactions Molecules that have permanent dipoles are attracted to each other.

Ion-Dipole Interactions

Attractive 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 Hydrogen Bonding • 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: Water

Hydrogen Bonding: Water