Why do dipoles form?

A dipole is the separation of two opposite charges, or, in this case, partial charges.

To answer your question, we have to distinguish between the different types of dipoles. There are three different types of dipole:

• Permanent
• Oscillating
• Induced

Permanent dipoles exist in molecules with covalent bonding where one atom is more electronegative than the other. The atom which is more electronegative attracts the bonded pair of electrons to it, increasing its electron density. It thus becomes slightly negative (#delta# negative). On the other end of the bond, the other atom loses electron density and becomes slightly positive (#delta# positive). The molecule now has a permanent dipole.

Oscillating dipoles occur by chance due to the random movement of electrons in atoms. At any point, the electrons in an atom can all be concentrated on one end, reducing the electron density of the other. This causes one end of the atom to become #delta# positive and the other to become #delta# negative - the atom now has dipoles. At another time, the electrons will be concentrated on the other end, so the dipoles will shift. The dipoles will constantly be shifting due to the random movement of electrons. This is called oscillating dipoles.

Induced dipoles form when a molecule with a permanent or oscillating dipole approaches a non-polar molecule (or the other way around). As the non-polar molecule approaches the polar one, its electrons will be attracted to the #delta# positive end of the molecule. Thus, a dipole has been induced into the non-polar molecule.

This sort of dipole can also form when a non-polar molecule approaches an ionic molecule.