As we’ve seen, molecules can carry three types of charges: 1.) they can be ions (with charge Q), 2.) they can be polar molecules with partial charges (determined by dipole moment μ), 3.) or they can have transient partial charges (i.e., induced dipoles) that result from distortion of their electron clouds (depending on polarizability α). The types of IMFs that exist between two molecules depend on the types of charges that exist in the two interacting molecules. Each pairwise combination of ions, dipoles, and induced dipoles are named according to the types of charges that are interacting. If two ions interact, we would call that an ion-ion interaction. If two polar molecules interact, we would call that a dipole-dipole interaction. If two molecules with induced dipoles interact, we would call that an induced dipole-induced dipole interaction (these are also called London dispersion forces). The other names on the slide represent all other combinations that exist. If an ion interacts with a polar molecule, it would be called an ion-dipole interaction. If an ion interacts with a molecule that has an induced dipole, it would be called an ion-induced dipole interaction. Finally, if a polar molecule interacts with a molecule with an induced dipole, it would be called a dipole-induced dipole interaction. The strengths of all of these interactions are dependent on the charge, dipole moment, and/or polarizability of the molecules that are involved.
In lecture we will also introduce a special type of dipole-dipole interaction known as a hydrogen bond. Hydrogen bonds are unique from other dipole-dipole interactions between they are unusually strong and have a directional component. Hydrogen bonds only form between molecules with specific structural features (we will explore these requirements in lecture).
The effects of IMFs are additive, and the overall strength of the IMFs in a substance is the sum of all of the IMFs that are present. In other words, some substances can make more than one of the interactions listed on the slide. For example, all substances, even polar ones, exhibit dispersion forces.