dipole moment
/ DY-pohl /
If a molecule has a slightly positive end and a slightly negative end, it behaves like a tiny bar magnet for charge — one side is plus, the other minus. The dipole moment is the measure of that separation: how much charge is split apart and how far. A molecule with a large dipole moment, like water, is strongly two-faced electrically; one with none, like methane, looks the same charge-wise in every direction.
More precisely, a dipole moment is a vector: it has both size and direction, pointing from the positive toward the negative end (chemists' convention), with magnitude equal to the amount of partial charge times the distance between the charge centers. It is measured in debye (D). Crucially, a molecule's overall dipole is the vector sum of all its individual bond polarities. Because it is a sum of arrows, bond dipoles can reinforce each other or cancel. Water's two O-H bond dipoles point partly the same way (the molecule is bent), so they add to a strong net dipole of about 1.85 D. Carbon dioxide's two C=O dipoles point exactly opposite (the molecule is linear), so they cancel to zero.
Dipole moments matter because they tell you whether a molecule is polar, and polar molecules behave very differently from nonpolar ones. Polar molecules attract one another through dipole-dipole forces, which raises their boiling points and lets them dissolve in polar solvents like water ('like dissolves like'). The honest subtlety worth repeating: you cannot judge a molecule's polarity from its bonds alone — you must add the bond dipoles as vectors, accounting for the molecule's three-dimensional shape, because symmetry can make a molecule full of polar bonds come out perfectly nonpolar.
Water (bent) has a net dipole moment of about 1.85 D and is strongly polar; carbon dioxide (linear, O=C=O) has equally polar bonds but a net dipole of 0 because they point opposite ways and cancel.
Same kind of polar bonds, opposite verdicts: shape decides whether the dipoles add or cancel.
Polar bonds do not guarantee a polar molecule. The net dipole is a vector sum, so symmetry can cancel everything out — judge polarity from the whole 3D shape, not bond by bond.