dipoles

Meaning: A dipole is a pair of equal and opposite partial charges (δ+ and δ−) separated by a distance. A bond dipole forms when two bonded atoms differ in electronegativity, so shared electrons sit closer to the more electronegative atom. The molecular dipole is the vector sum of the bond dipoles. Polar molecules attract one another through dipole-dipole forces, and hydrogen bonding is an especially strong form of this attraction.

Equation: μ = q × d, where μ is the dipole moment (C·m), q is the magnitude of the separated charge (C), and d is the distance between the charges (m).

On the MCAT: Draw the Lewis structure, use VSEPR to find the shape, and decide whether the bond dipoles cancel. Then predict properties: polar molecules tend to dissolve in polar solvents and often boil higher than nonpolar molecules of similar mass. A classic comparison is cis- versus trans-1,2-dichloroethene; the cis isomer has a net dipole and the trans isomer does not.

Common confusion: Polar bonds do not guarantee a polar molecule. CO₂, CCl₄ and BF₃ have polar bonds but are nonpolar because their symmetric shapes cancel the dipoles. H₂O and NH₃ are polar because their lone pairs make the shapes asymmetric.