Permanent dipole-dipole interactions

When the electronegativity difference in a chemical bond is significant, the electron density is strongly skewed, which produces a permanent dipole. Permanent dipole-dipole forces arise when the negative part of one dipole attracts the positive part of another dipole.

The strength of permanent dipole-dipole forces depends on the strength of each individual dipole and the geometry of the molecule. An example of how geometry plays a role is the fluoromethanes.

Examples of compounds with permanent dipoles include halogen methanes, ketones like acetone, and water. Water contains a specific subcategory of permanent dipole-dipole forces known as hydrogen bonding.

Figure 1: Diagram showing how the molecular dipoles in two atoms give rise to permanent dipole-dipole forces