dipole moment

When two atoms of different appetites for electrons form a bond, the electrons sit closer to the greedier one, leaving it slightly negative and its partner slightly positive. This lopsided charge is a tiny built-in arrow from positive to negative — a dipole. The dipole moment measures how big that separation of charge is, and it explains why some molecules are sticky and water-loving while others are not.

Quantitatively, a dipole moment is the size of the charge separation multiplied by the distance over which it is separated, and it is a vector with a direction (conventionally pointing toward the negative end). It is commonly measured in debye units. A single polar bond has a bond dipole; the molecule's overall dipole is the vector sum of all its bond dipoles plus any lone-pair contributions. This is where shape becomes decisive: if the bond dipoles point in opposing directions and cancel, the molecule is nonpolar even though its individual bonds are polar.

Dipole moments connect electronegativity and geometry to the properties you can feel. CO2 has two polar C=O bonds, but because the molecule is linear they point in exact opposition and cancel, so CO2 is nonpolar. Water has two polar O-H bonds, but its bent shape means they do not cancel, so water has a large dipole — the source of its high boiling point and its power as a solvent for ions. Comparing the dipoles of similarly-shaped molecules is one of the cleanest ways to see electronegativity differences in action.