Intermolecular Forces

In some ways, a collection of gas molecules represents the simplest form of matter. Because the individual molecules are so far apart, they have only fleeting interactions with one another. In contrast, molecules that have clustered together to form a liquid or solid are constantly exerting forces on each other. In fact, it is only because of these attractive forces that molecular solids and liquids exist at all. In this lesson, we will look at some of the ways in which molecules and ions attract one another to form solids and liquids.

Ion-Ion Interactions Ion-ion interactions have already been considered in a previous chapter, so we will simply do a short review. These interactions are most significant in the solid state. When dissolved in water, ions are shielded from one another by water molecules, making ion-ion interactions less prevalent.

In the solid state, ions interact by forming lattices in which oppositely charged ions arrange themselves in a regular fashion. In this image, the small purple Na ions interlock with the larger green chlorine ions in a pattern defined by the relative sizes and charges of the two ions. Because each ion has a full positive or negative charge, the forces holding two ions together are relatively strong.

More complex polyatomic ions can also participate in these types of interactions. Sodium acetate is one such material.

Dipole-Dipole Interactions A dipole occurs when two opposite charges are separated by some amount of distance. We have already seen dipoles in the form of polar bonds. For example, each O-H bond in water is an example of a dipole; the partial positive charge on hydrogen is separated from the partial negative charge on oxygen by the length of the bond. A molecular dipole is the geometric sum of all the individual bond dipoles in a molecule. In order for a molecule to have a dipole, it must have at least one polar bond. However, not all molecules with polar bonds have an overall molecular dipole. Sometimes the dipoles within a molecule will effectively cancel each other out, giving a zero net molecular dipole. This is often seen in symmetrical molecules.

A dipole-dipole force can be seen when two polar molecules interact with one another. The partial positive region of one molecule will be attracted to the partial negative region of an adjacent one. Because both charges are partial (less than a full charge), these interactions are weaker than those involving ions. An example of a dipole-dipole interaction between two molecules of HCl is shown.

Ion-Dipole Interactions An ion-dipole interaction involves the attraction between a fully charged entity and a polar molecule. Both cations and anions can participate in this type of bonding. A cation will be attracted to the partial negative portion of the polar molecule, while an anion will interact with the partial positive region.

The interaction of sodium and chloride ions with water is one example of an ion-dipole interaction: The positive sodium ions are attracted to the partial negative portion of the water molecule (the red oxygen atoms), while the negative chloride ions interact with the partial positive hydrogen portion (the blue atoms). Because one of the bonding partners has only a partial positive or negative charge, these forces are somewhat weaker than ion-ion interactions.

Dispersion Forces Somewhat more challenging to visualize are dispersion forces. These interactions are defined as attractive forces that arise as a result of temporary dipoles induced in atoms or molecules. They are often referred to as London dispersion forces due to the work done by the German-American physicist Fritz London (1900-1954). Two things need to be kept in mind with regard to these forces: they are relatively weak, and they do not require any permanent polarity. Induced dipoles are caused by local and temporary changes in the environment immediately around a molecule. Brief distortions in the electron cloud cause temporary dipoles to come and go, and these provide a way for even completely nonpolar molecules to attract one another. Although these may seem almost insignificant compared to the stronger forces discussed above, nonpolar substances would have no way to form solids or liquids without them. Note that all molecular substances experience dispersion forces, but for small polar molecules, dipole- dipole interactions will be the dominant attractive force.

The Hydrogen Bond A “special case” of dipole-dipole interactions is referred to as the hydrogen bond. Hydrogen bonding occurs only in molecules where hydrogen is covalently bonded to one of three elements: fluorine, oxygen, or nitrogen. These three elements are so electronegative that they withdraw the majority of the electron density from the covalent bond with hydrogen, leaving the H atom very electron-deficient. Because the hydrogen atom does not have any electrons other than the ones in the covalent bond, its positively charged nucleus is almost completely exposed, allowing strong attractions to other nearby lone pairs. These lone pairs are generally on atoms with partial negative charges in adjacent molecules, although hydrogen bonds within a single molecule can also occur if the structure of the molecule is appropriate.

A particularly important example of hydrogen bonding occurs between water molecules. Because water has two O-H bonds and two lone pairs on each oxygen atom, extensive networks of hydrogen bonds can form, allowing ice and liquid water to exist.

Quite often, the partial charges are not explicitly written when drawing hydrogen bonds, but they are always there. Here are a few more examples of hydrogen bonding in molecules.

Lesson Summary •A variety of interactions can occur between molecules that involve attractions between full or partial charges. •Molecules that have a partial positive region and a partial negative region are said to possess a molecular dipole. The interactions between these dipoles are what allow molecules to condense into the liquid or solid states. •Even completely nonpolar molecules can attract each other due to dispersion forces. •The hydrogen bond is a special type of dipole-dipole interaction that is seen in a variety of molecular com- pounds.