The force of attraction of a dipolar molecule for another molecule or ion depends on how much charge is separated and how great the separation is. The strength of a dipole is measured by its dipole moment, u. If two equal and opposite charges, +q and -q, are separated by a distance r, then the dipole moment is defined as

u = qr

Either doubling the charges, or doubling the separation between them, has the effect of doubling the strength with which the molecule will attract its neighboring molecules. The dipole moment of a molecule is an easy quantity to measure and is a useful indication of a molecule's chemical and physical behavior.