2.4 Intermolecular Forces

Intramolecular Forces: – the attractive forces between atoms within a

molecule– the attractive forces between ions within an

ionic crystal– e.g. ionic bond, covalent bonds– strong

Intermolecular Forces:– the attractive forces between molecules– e.g. Van der Waals forces and hydrogen bonds – weak (in comparison to intramolecular forces)– i.e. much less energy to melt H2O (inter) than for

it to decompose into H2 and O2 (intra)

Intermolecular Forces• if covalent bonds were the only forces at

work, most molecular compounds would be gases as there would be no attraction between molecules strong enough to group the molecules as liquids or solids

van der Waals Forces

• Dipole-dipole• London dispersion

Dipole-dipole• forces of attraction between oppositely

charged ends of polar molecules.• the strength of the dipole-dipole force is

dependent on the polarity of the molecule

London Dispersion

• attractive forces between all molecules, including nonpolar molecules

• result of temporary displacements of the electron cloud around atoms in a molecule (extremely short-lived dipoles)

• therefore weaker than dipole-dipole

• strength depends on number of electrons in a molecule

Hydrogen Bonding• strong dipole-dipole force

between the positive H atom of one molecule and the lone pair of electrons on the negative atom (N, O, or F) on another molecule

Properties of Gases and Liquids

• very weak intermolecular forces acting on gases

• in liquids, intermolecular forces account for some special properties.

Surface Tension

• molecules in a liquid are attracted by molecules on all sides

Cohesion and Adhesion• Cohesion: The attraction between like molecules• Adhesion: The attraction between unlike molecules.

Adhesion between water and glass is higher than cohesion between water

molecules.

Cohesion between mercury is higher

than adhesion between mercury

and glass