How do dipoles form?
Differing electronegativities between atoms within a molecule result in a dipole.
Different atoms have different abilities to attract electrons to their nucleus. As the size of the nucleus increases going from left to right across the periodic table, it adopts more protons. Protons are positively charge, so the nucleus will become more positively charged. Electrons are negatively charged, and are more attracted to a more positively charged nucleus. The ability of an atom's nucleus to attract electrons is called its electronegativity .
To show how a dipole could arise from this phenomenon, think of the difference between an -O-H bond and an H-H bond. In both bonds, two electrons are shared between the atoms. However, in the O-H bond, the O is more electronegative than the H, meaning it tends to attract the electrons in the bond more than the H does. This difference in electron distribution is what is known as a dipole . In the H-H bond, both H's have the same tendency to attract electrons, and, thus, there is no dipole.
The net dipole of a molecule, is the vector sum of all of the dipoles present in a molecule.