8 - 1 Intermolecular Forces Intermolecular forces are attractive forces between molecules. The type of intermolecular forces present dependent upon:  the types of bonds within the molecule itself.  the geometry or the three dimensional shape of the molecule.

8 - 2  the resulting polarity of the molecule. Three types of intermolecular forces are:  dispersion forces  dipole forces  hydrogen bonding These three forces are relatively weak compared to the forces found in a covalent bond.

8 - 3 van der Waals Forces Also known as London dispersion forces. van der Waals forces are found in all molecular (covalent) compounds which result from temporary (induced) dipoles. Temporary dipoles are caused by the localization of electrons in an atom or molecule. Nonpolar covalent have only dispersion forces.

8 - 4 Two nonpolar molecules are shown below. These two molecules at an instant of time are not polarized and therefore experience no attraction.

8 - 5 Two nonpolar molecules are shown below. These two molecules at an instant of time are polarized and therefore experience an attraction. Molecule A induces a similar dipole to result in molecule B. δ-δ- δ+δ+ δ-δ- δ+δ+ A B

8 - 6 This polarization is caused by the shifting of the electron charge density due to electrons repelling each other and an electron and a nucleus attracting each other. van der Waals forces are weak forces of attraction between nonpolar molecules. The strength of these induced dipole forces depend upon:  The number of electrons making up the molecule.

8 - 7  The relative ease of localizing the electrons.  Both of these factors increase with increasing molecular size.  Larger atoms have a weaker attraction for their valence electrons making it easier to displace these electrons.

8 - 8 Dipole Forces Dipole forces exist between polar molecules. A molecule is polar as a result of the unequal sharing of electron pairs between covalently bonded atoms. The major distinction between dipole forces and van der Waals forces is that they are permanent forces. δ-δ- δ+δ+ δ-δ- δ+δ+

8 - 9 Polar covalent molecules align themselves such that the positive end of one molecule is close to the negative end of another molecule. Dipole forces are dependent on the difference of electronegativities between the atoms. Polar covalent molecules have both dispersion and dipole forces and generally have higher melting/boiling points than nonpolar compounds.

Hydrogen Bonding Hydrogen bonding among water molecules.

The previous slide illustrates the attractive forces between the highly polarized molecules containing hydrogen. Hydrogen bonding results when a hydrogen atom is bonded to nitrogen, oxygen, or fluorine and there is/are unshared pair(s) of electrons of neighboring molecules. When hydrogen is bonded to oxygen, the small size of the hydrogen atom allows it to draw very closely to the unshared pair of electrons oxygen.