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Intermolecular Forces, Solids and Liquids 159
strength and type of intermolecular forces present. Before we begin examining
the different types of intermolecular forces, you may wish to skim Chapter 9 on
bonding. It is important to recognize which molecules are polar and which are
not. Polar molecules have partial positive and partial negative ends. Polar molecules
have a dipole. These dipoles are one of the major factors involved in the
intermolecular forces.
Covalent, metallic, and ionic bonds are very strong interactions. Some people
consider these to be intermolecular forces. The following are weaker intermolecular
forces. They appear in approximate order of decreasing strength. Even
though weaker than bonds, they are nonetheless important.
Don’t Forget!
Intermolecular forces occur between molecules not within molecules. For example,
covalent bonds hold a water molecule together, but intermolecular forces,
not covalent bonds, attract separate water molecules to each other.
Quick Tip
Ion-Dipole Intermolecular Forces
This attraction is due to the attraction of an ion (cation or anion) with one end
of a polar molecule (dipole). This type of attraction is especially important in
aqueous salt solutions where the ion attracts water molecules and may form a
hydrated ion (i.e., Al(H 2O) 6 3 ).
The ion-dipole intermolecular force is different; in that two different species
must be present: an ion from one species and a polar molecule from a different
species.
Dipole-Dipole Intermolecular Forces
This attraction occurs between two polar molecules. It results from the attraction
of the positive end of one dipole to the negative end of another dipole. For