Intermolecular

Forces

IM/VdW forces are the forces of attraction between molecules – not

within the molecule.

VdW forces vary in strength depending on the interactions between

molecules.

Van der Waals Forces

aka…

Intermolecular

ForcesDipole-Dipole

Weaker force…

Polar molecules act as tiny “dipoles” (equal & opposite charges separated by short

distances)Dipole – dipole forces attract between

molecules such as between two water molecules.

Positive H region is attracted to negative O region of a different molecule.

attracted to (like a magnet)

Electrons of one atom are attracted to another atom’s

nucleus

d-d+

Which of the following are polar molecules? Where are their dipole

regions?

O

HH

N

HH

H

CCO O

Al

Cl

Cl

Cl

Cl

Cl

Cl

ClS

HH

Hydrogen bondingIn hydrogen bonding, a hydrogen atom is

attracted to an unshared pair of electrons of an –OH, -NH, or –FH group.

Stronger

force…

Intermolecular Forces

Surface TensionEnhancement of the intermolecular attractive forces at the surface

Hydrogen bondingEvidence…

What causes surface tension?

The cohesive forces between molecules are shared with all neighboring atoms.

Since the surface has no neighboring atoms above, they exhibit stronger attractive forces for their neighbors next to and below them

Surface tension is a result of cohesive intermolecular forces

How is surface tension affected by soap?

Soap breaks the surface tension!