CHE 170 – Mueller – INTERMOLECULAR FORCES
Molecular Compounds:
It is important to point out the difference between the covalent bonds and intermolecular forces.
Covalent bonds exist within molecules. The atoms are sharing electrons, so must stay together as a
molecule. Bonds within molecules are quite strong. Intermolecular forces are attractions between
molecules and are relatively weak. These forces bring separate molecules together to make liquids and
solids.
Melting and boiling are good examples of the difference. When melting ice (solid H2O), water is
formed (liquid H2O). The strong bonds within water are not broken when melting; it is the weaker
intermolecular forces between molecules that are overcome. Similarly, when boiling water (liquid
H2O), steam is formed (gaseous H2O). The bonds are not broken, it is again the intermolecular forces of
attraction that are overcome.
Three Main Types of Intermolecular Forces
(These weak forces between molecules are also known as Van der Waals Forces.)
London-Dispersion Forces (between all molecules):
When molecules are close to each other, the electrons from each repel and slosh over to one side. If the
electrons are more on one side, it makes that side partially negative and the other side partially positive
inducing a temporary dipole moment. (The dipole moment is often called “instantaneous” and
“induced” because it only exists temporarily, caused when the molecules are close.) The partial
negative end of one molecule will be attracted to the partial positive end of another molecule. Larger
molecules with more electrons are called more polarizable because their larger electron clouds can slosh
more, creating larger temporary dipole moments and thus stronger London-Dispersion Forces.
Dipole-Dipole Forces (between polar molecules):
Polar molecules have permanent dipole moments. The partial negative end of one molecule will be
attracted to the partial positive end of another molecule.
Hydrogen Bonding Forces (between molecules that have H directly bonded to N, O, or F):
The bond between hydrogen and nitrogen, oxygen, or fluorine is unusually polar. (N, O, and F
are the three most electronegative elements, so they pull electrons the most and will be super
partially negative. Hydrogen only has one electron, so if that electron is pulled away, there is a
just a bare, naked proton left behind that will be super partially positive.) Molecules with this
type of super polar bond with have super strong dipole-dipole forces; significant enough to have
their own category. Note: this is still a type of intermolecular force of attraction between
molecules which happens to result from a type of bond within the molecule.
Ionic Compounds:
Negative ions are not sharing electrons with positive ions; they instead stay together due to the attraction
of opposite charges. This also means that a negative ion is attracted to all of the nearby positive ions,
not just one in particular. Ionic compounds do not form “molecules”; each ion is not just bonded with
another specific ion --- all the negatives are attracted to all the positives. Thus the forces between (inter)
and the bonds within (intra) are really the same thing in ionic compounds. Ion-ion forces and ionic
bonds are the same thing --- both very strong. This is why ionic compounds are solids with very high
melting points --- very strong attractions, so it takes a very high temperature to break them apart.
Using Intermolecular Forces to Predict Physical Properties
Substances with stronger intermolecular forces will have
 higher melting points and higher boiling points. (If stronger forces of attraction, must get to
higher temperature to break apart.)
 lower vapor pressure (less will vaporize to gas phase – more likely to stay stuck together as
liquid or solid).
 higher viscosity and higher surface tension (stick together more).
How to Predict Relative Strength of Intermolecular Forces
If the compound is ionic (metal & non-metal; electronegativity difference of 2.0 or more) the ion-ion
forces are the same as the very strong ionic bonds. Ionic Compounds have the Strongest Forces. (This
is why KCl has a higher melting point that ClBr.)
If the compound is molecular (covalent bonds; non-metal & non-metal; electronegativity difference of
less than 2.0), first look at the types of forces. If the substance has more types of forces then, overall,
the forces will be stronger. (HF has a higher boiling point that HCl because HF has all three types of
forces, but HCl only has London-Dispersion and Dipole-Dipole Forces.)
If the molecular compound has the same types of forces, the larger molecules will have stronger overall
forces. Larger molecules are more polarizable, so stronger London-Dispersion forces. (This is why
CCl4 has a higher melting point that CF4.)
Stronger Forces if (1) Ionic; (2) More Types; (3) Larger Molecules; 4) More Surface Area
*These arguments apply only to ionic and molecular compounds
(not metallic or long-range covalent crystals).
“Like Dissolves Like” – An ionic compound, with full positive and negative charges, will only
dissolve in very polar liquids. Molecules that have hydrogen-bonding and dipole-dipole forces
are the most polar; with the largest permanent partial charges.
Ions will not dissolve in a non-polar liquid (which only has the temporary, induced dipoles of
London-dispersion forces). See pages 422, 469, & 472 in Tro.