Untitled - Kelly Walsh High School

160 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
example, gaseous hydrogen chloride, HCl(g), has a dipole. The hydrogen end
has a partial positive charge and the chlorine end has a partial negative charge.
The chlorine is more electronegative so it has the partial negative charge. The
partial-positive end of one HCl molecule attracts the partial-negative end of
another HCl molecule. Dipole-dipole attractions are especially important in
polar liquids. These dipole-dipole attractions tend to be rather strong intermolecular
forces, although not nearly as strong as ion-dipole attractions.
Hydrogen Bonding Intermolecular Forces
Hydrogen bonding is really a subtype of dipole-dipole attraction. In this case, a
hydrogen atom bonds to a very electronegative element. The only elements sufficiently
electronegative are: N, O, or F. The resultant bond to hydrogen is more
polar than the electronegativity difference would predict. This extreme polarity
leads to a greater than expected degree of charge separation. Therefore, the
attraction of the hydrogen of one molecule and the N, O, or F of another molecule
is unusually strong. Hydrogen bonds tend to be stronger than the typical
dipole-dipole interaction.
Hydrogen bonding explains why water has such unusual properties. We will discuss
these properties later in this chapter.
Hydrogen bonding can only occur when a hydrogen atom is bonded directly to
an N, O, or F.
Ion-Induced Dipole and Dipole-Induced Dipole
Intermolecular Forces
These types of attractions occur when the charge on an ion or a dipole distorts
the electron cloud of a nonpolar molecule. This induces a temporary dipole in
the nonpolar molecule. These are fairly weak interactions. Like an ion-dipole
force, this type of force requires the presence of two different substances.
London (Dispersion) Intermolecular Forces
This intermolecular attraction occurs in all substances. It is usually only significant
for nonpolar substances. It arises from the momentary distortion of the
electron cloud. This distortion causes a very weak temporary dipole, which