The Forces Between Molecules

All matter is held together by force. The force between atoms within a molecule is a chemical or intramolecular force. The force between molecules is a physical or intermolecular force. We learned about intramolecular forces and the energy it took to overcome these forces, earlier in our chemical studies. Now we will focus on intermolecular forces.


The Nature of Intermolecular Forces:

The Intermolecular Forces (forces between molecules) are weaker than Intramolecular Forces (The Chemical Bonds within an Individual Molecule). This distinction is the reason we define the molecule in the first place. The properties of matter result from the properties of the individual molecule (resulting from chemical bonding) and how the molecules act collectively (resulting from intermolecular forces).

Intermolecular Forces are longest-ranged (act strongly over a large distance) when they are electrostatic. Interaction of Charge Monopoles (simple charges) is the longest-ranged electrostatic force.
Charge-Charge forces (found in ionic crystals)

For like charges (+,+) or (-,-), this force is always repulsive. For unlike charges (+,-), this force is always attractive.

Charge-Dipole Forces:
An uncharged molecule can still have an electric dipole moment. Electric Dipoles arise from opposite but equal charges separated by a distance. Molecules that possess a dipole moment are called Polar molecules (remember the polar covalent bond?). Water is polar and has a dipole moment of 1.85 Debye. The Debye is a unit of dipole moment and has a value of 3.336 x 10-30 Coulomb meter.

When salt is dissolved in water, the ions of the salt dissociate from each other and associate with the dipole of the water molecules. This results in a solution called an Electrolyte

The force may be understood by decomposing each of the dipole into two equal but opposite charges and adding up the resulting charge-charge forces. Notice that the Charge-Dipole Forces depend on relative molecular orientation. This means that the forces can be attractive or repulsive depending on whether like or unlike charges are closer together. On average, dipoles in a liquid orient themselves to form attractive interactions with their neighbors, but thermal motion makes some instantaneous configurations exist fleetingly that are, in fact, repulsive.

Dipole-Dipole forces exist between neutral polar molecules. Again, this force may be understood by decomposing each of the dipole into two equal but opposite charges and adding up the resulting charge-charge forces.



The following table demonstrates the effect of the dipole moment on the boiling point of several substances:
 
 

Substance
Molecular Mass
[g/mol]
Dipole moment
[Debye]
Normal Boiling Point
[ K ]
Propane
44
0.1
231
Dimethyl ether
46
1.3
248
Chloromethane
50
2.0
249
Acetaldehyde
44
2.7
294
Acetonitrile
41
3.9
355

Electrostatic forces are defined (categorized) by the symmetry of the partners involved in the interaction. This symmetry is labelled by the first non-zero moment of the charge distribution, i.e. Monopole, Dipole, Quadrupole, etc. Electrostatic forces only exist between molecules with permanent moments of their charge distribution; Molecules do not have to distort or fluctuate in order to exhibit electrostatic intermolecular forces.

Electrostatics cannot explain the whole story, however. Molecules that are round and have no charge have no electrostatic forces between each other. How, then, do round molecules form liquids or solids?


Inductive Forces and Dispersion

Inductive forces arise from the distortion of the charge cloud induced by the presence of another molecule nearby. The distortion arises from the electric field produced by the charge distribution of the nearby molecule. These forces are always attractive but are in general shorter ranged than electrostatic forces. If a charged molecule (ion) induces a dipole moment in a nearby neutral molecule, the two molecules will stick together, even though the neutral molecule was initially round and uncharged:

Other inductive forces exist (permanent dipole - induced dipole, etc.) but this one (charge-induced dipole) is the strongest.


Inductive forces that result not from permanent charge distributions but from fluctuations of charge are not called inductive forces at all but are called London Dispersion forces. These forces are ubiquitous but are most important in systems that have no other types of molecular stickiness, like the rare gases. The rare gases may be liquified, and it is dispersion forces that hold the atoms together (no electrostatic or inductive forces exits)



The movement of the electrons, even in the He atom, cause an instantaneous dipole to be formed. The time-averaged dipole moment of the atom is still zero. This dipole, however fleeting, can induce a dipole in a neighboring atom, causing a force. This force is always attractive but even shorter ranged (and weaker) than permanent dipole-induced dipole forces.

Size (Volume and Shape) determines the magnitude of the dispersion force. The bigger the size, the larger the dispersion force.


Hydrogen Bonding



The figure above shows the normal boiling point temperatures for several related substances. This boiling point diagram tells us about the intermolecular forces between a homologous series of small hydrogen containing molecules. Look first at the Group IV hydrides, from CH4 through SnH4. The boiling points of these molecules increase with increasing mass, as one would expect. The group VI hydrides do the same thing, with the notable exception of WATER! A special type of intermolecular force exists between water molecules called hydrogen bonding, which raises its boiling point significantly with respect to its isovalent homologs.

Hydrogen is unique among the elements because it has a single electron which is also a valence electron. When this electron is hogged by another atom in a polar covalent bond, a significant fraction of the hydrogen nucleus becomes uncovered and the bare nucleus desperately seeks to be covered by electrons from other atoms (modesty?).

A Hydrogen Bond is the attractive interaction between two closed shell species that arises from the link of the form A-H...B, where A and B are highly electronegative elements and B possesses a lone pair of electrons. Normally, hydrogen bonds only exist when atoms A and B are Nitrogen, Oxygen, and Fluorine. If the element B is anionic (such as Cl-) and thus a very good electron donor, it may also participate in hydrogen bonding. Hydrogen bonding is very important the the function of proteins, as these interactions determine the way they fold (their shape), and this determines how they react in the cell. Fluorine hydrogen bonds not found too often in biochemistry, but can be important in certain synthetic materials properties.


Summary of Types of Intermolecular Forces





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